Tooth decay has been cut by an astounding one-third in American children in the past decade, thanks mainly to fluoridated water supplies and fluoridated toothpaste.

Fifty-nine percent of children aged 5 to 11 had never had a cavity in the latest nationwide survey by the National Institute for Dental Research.

Among youngsters 5 to 17, 37 percent were decay-free, according to this 1979-80 study. In 1971-73 the number was only 28 percent.

By another measure, the average child aged 5 to 17 had 7.06 decayed, missing or filled teeth in 1971-73 but only 4.77 at the end of the decade.

These cheerful figures were presented to the American Association for the Advancement of Science here yesterday by Dr. Janet Brunelle, chief of biometry in the dental institute, part of the National Institutes of Health.

The news is cheerful because tooth decay is still the leading chronic disease of childhood, and it costs the nation at least $5 billion a year.

The news about teeth is not all bright, however. Only 17 percent of 12- to 17-year-olds were caries-free in the last survey. Chances are, said Brunelle, that their exposure to sugar in drinks, foods and snacks had just been longer than the younger children's. Also, many had not been exposed for as long to either fluoridated water or fluoride toothpaste.

More and more communities have started fluoridating their water in the last 30 years, but they still account for just over half the population. Fluoride toothpastes are far newer, but already command some 80 percent of the market.

Brunelle also credited fluorides with combatting decay two other ways. Dentists prescribe fluoride to be added to children's diets, and schools give youngsters fluoride tablets for daily or weekly use. She also said that some families have begun to emphasize less consumption of sugar and sugary products.

Some new dental methods promise to cut decay even further.

NIH and other federally funded scientists are working on anti-decay vaccines.

Dr. L.M. Silverstone of the University of Iowa told the AAAS of another technique that a growing number of dentists are using: application of a plastic sealant to children's teeth to stave off the gunk that causes decay. The dentist etches the enamel surface with a harmless acid, then applies a "fissure sealant," an artificial resin or plastic.

In one study, 78 percent of sealed teeth but only 22 percent of teeth left unsealed stayed decay-free for five years.

If the sealant is applied by dental hygienists or assistants--perfectly acceptable, said Silverstone--the cost need only be about $1.75 a tooth. Three-fourths of sealed teeth retain the sealant 12 to 18 months. The use of the sealants, he added, can cut a youngster's total dental bill at least in half.