Ear infections, a misery that afflicts more than 70 percent of American children under 3 years old, have taken on new and troubling significance. Doctors believe the fluid left behind by infections may affect speech development and intelligence by intefering with hearing at the age when children learn to talk.

There were 30 million visits to doctors' offices for such infections last year, at an estimated total annual cost of $2 billion -- counting doctors' fees, antibiotics and various kinds of surgery.

Because of concern about fluid in the ear, operations to insert tiny tubes in toddlers' eardrums have almost doubled in a decade, and now rival or surpass tonsillectomies as one of the operations most often done on children. As many as 1 million American children have tubes placed in their ears each year to drain the fluid and restore normal hearing, according to one expert who based his estimate on sales data from tube manufacturers.

The need for the surgery has become increasingly controversial, however, with some doctors arguing that for many children, less drastic and costly measures might be equally effective.

Despite the commonness of ear infections, there are suprising gaps in medical understanding of their causes and complications. Some children have earaches month after month in their early years. Other have none. No one is sure whether the difference reflects genetics, skull structure, allergies or impaired defenses against bacteria.

Nor is it known whether a toddler whose hearing is temporarily reduced -- either during episodes of infection or in the weeks following, when fluid may remain in the ear -- suffers any permanent harm. Depending on the answer, doctors may be operating to insert tubes far too often, or not often enough.

"If a child has multiple episodes, he could spend a fair proportion of the first two years of life with some impairment of hearing," said Dr. Jerome O. Klein, a professor of pediatrics at Boston University Medical School. " . . . These children have to hear sound through an ocean. If it is shown that there is a [resulting] disability that is permanent . . . we'll have to be much more aggressive in getting that fluid out."

When Klein tested speech in a group of 3-year-olds who had had periods with fluid in the ear, he found they scored significantly lower than a group who had not. Another study by Alabama pediatricians found a lower range of I.Q.s among second-graders who had had multiple ear infections in the first year of life, compared with others who had not.

Klein plans to test his subjects again at age 7, to see if speech development will catch up. He also wants to determine whether fluid in the ear is harmful if it is present at some unknown critical age.

"It is possible that a child at a particular time of life will have severe, irremediable damage -- for instance, if you had fluid in your ears between 3 and 12 months, whereas if you had fluid in your ears between 2 and 3 years you may have no damage," he said.

It is because of worries about lasting impariment that surgery to insert tubes in children's eardrums has become so frequent. But it is only one of many treatments being used for the condition, including antibiotics, decongestants, balloons and similar devices to open the Eustachian tubes, adenoidectomy with or without tonsillectomy, or simply incising the eardrum to drain fluid.

In children, tube insertion -- done by an operation called myringotomy -- is usually performed in a hospital under general anesthesia. The surgeon cleans wax from the ear canal, sterlizes the area, then makes a tiny incision in the eardrum. After sucking out fluid from behind the drum, the surgeon inserts a plastic tube -- shaped like a bobbin or collar button -- into the hole. This maintains an artificial perforation in the eardrum until the tube falls out, normally six months to a year later.

According to Dr. Kenneth M. Grundfast, the chief ear, nose and throat specialist at Children's Hospital, the operation carries certain risks: the risk of anesthesia itself, the small but ever-present chance of a surgical mishap, and the slight possibility that the hole made to insert the tube will not heal after the tube comes out. At Children's Hospital, a child having the surgery is admitted for only a few hours, and the total charge averages between $750 and $850.

Doctors most often recommend the operation for one of two reasons: to provide a permanent drainage path for fluid that has collected behind the eardrum and that has reduced hearing, or to prevent ear infections in a child who has had them repeatedly despite other treatments. Grundfast said there is much more evidence backing the first reason that the second, although he believes both are valid.

Some doctors, he said, may be mistakenly rushing children into ear tube surgery rather than using other effective treatments. But he also believes there are certain children who definitely need ear tubes, and does not want to see the operation fall into disrepute from overuse as occurred with tonsillectomies.

"It [myringotomy] is in vogue and possibly being overdone," he said. "The advantage is that if a child is not hearing properly, the tube, as opposed to any medical therapy, immediately provides improved hearing." But he added, "I would not want it to be the first thing done."

Young children are vulnerable to ear infections, and slow to recover from them, because they easily develop blockage of the Eustachian tube, a passage that runs from the throat to the middle ear, the chamber that lies behind the eardrum.

Within the middle ear are structures essential to good hearing. Three small bones -- the malleus, incus and stapes -- are arranged in a chain to transmit vibrations from the eardrum (a taut, delicate membrane stretched across the ear canal) to the cochlea, a conch-shaped structure filled with fluid. Hair-like structures inside the cochlea respond to movement of the fluid, in turn stimulating the auditory nerve, and sound is heard.

Since the eardrum and bones must vibrate freely in response to sound, Grundfast said, good hearing depends on having only air in the middle ear. "The only way you can get air into the space is through the Eustachian tube," he said. Normally the opening to the Eustachian tube is closed, but it opens, admitting air to the middle ear, when a person yawns or swallows.

Children most often develop temporary blockage of the Eustachian tubes between the ages of 6 months and 2 years. This tendency depends partly on the shape of the skull, which determines the length and position of the tube, and may explain why ear infections are more common in Eskimos and Indians than in Caucasians and blacks. Some doctors also believe large adenoids can block the tube, and advocate adenoidectomies for children prone to ear infections.

After a child is 2 or 3 years old, he usually gets fewer ear infections because the Eustachian tube grows and the child's immunity to viruses and bacteria improves, according to Boston University's Klein. He said there is a brief increase in ear infections among 5- and 6-year-olds, who are exposed to unfamiliar germs when they enter school.

During an infection, bacteria multiply in the middle ear, and the child develops a fever and severe pain because of pus pressing on the eardrum. Hearing is also temporarily affected. Because of the pain and fever, the child is taken to the doctor, who usually prescribes antibiotics. The infection then subsides gradually over several days, but fluid may stay in the ear -- sometimes for weeks -- if the Eustachian tube remains blocked.

Klein, who has studied ear infections in about 2,500 Boston children, said that by age 3, one-third of children have had between one and three ear infections, and another third have had more than three.

Dr. Charles D. Bluestone, professor of otolaryngology at the University of Pittsburgh medical school, said infections have become much more common in the last 20 years. He suspects that it may be because doctors, in their eagerness to threat the illness and prevent complications, are prescribing antibiotics too freely.

The use of antibiotics has decreased the freqency of the most serious complications of ear infections, such as meningitis and mastoiditis. But it has not reduced the chance that fluid will remain in the ear after an infection heals.

Klein has found that 70 percent of children still have fluid in the middle ear when they finish taking antibiotics for an infection. A month after the infection, fluid persists in 40 percent, and two months after, in 20 percent. After four months, 5 percent still have fluid in the ear.

Bluestone's estimate that 1 million children receive tubes each year is higher than figures from the National Center for Health Statistics, which recorded 223,000 myringotomies in children -- most done for tube insertion -- in 1979. But the center's figure does not include tubes inserted in patient's in doctors' offices or outpatient facilities of hospitals.

Bluestone is comparing the effectiveness of surgery and alternative treatments at a research center at Pittsburgh's Children's Hospital devoted exclusively to ear infections. He has already reached preliminary conclusions about some. For example, he believes decongestants and adenoidectomies do not help drain fluid from children's ears and that antibiotics probably do.

Tube insertion "is a very valid procedure when properly applied," said Grundfast of Washington's Children's Hospital. "If a child is going to learn to speak properly, he has to be able to listen clearly and imitate the sounds he hears."

But he added, "I think the best thing . . . is for people to think very distinctly about which individuals need it, and to give all other methods of management a try before inserting a tube."