Quitting smoking yields "major and immediate health benefits," regardless of a smoker's age, according to the surgeon general's annual report on the health consequences of smoking, which was released yesterday.

Smoking-related illnesses cause more than one out of every six deaths in the United States, according to the report. Although heart disease, stroke, chronic lung disease, many cancers and a variety of other disorders are more common in smokers than in nonsmokers, the report found that the risk of these health problems begins to decline as soon as a smoker quits.

Someone who stops smoking before age 50 is only half as likely to die within the next 15 years as a person who continues to smoke, according to the report, which is the first to focus entirely on smoking cessation.

"The earlier one quits, the greater the benefits," said Surgeon General Antonia C. Novello. But, she added, "even people who quit smoking at older ages can expect to enjoy a longer and healthier life compared with those who continue to smoke."

More than 38 million Americans have quit smoking, and 90 percent of those did so on their own, according to the report. But private health insurance plans, state Medicaid programs and perhaps the federal Medicare progam should cover the cost of smoking cessation programs for smokers who need extra help, said William L. Roper, director of the federal Centers for Disease Control. Most insurance plans do not currently pay for such programs.

The 628-page report summarized the results of hundreds of studies on smoking-related illnesses and on how their frequency is affected by quitting. It concluded that for some diseases, an ex-smoker's risk of illness declines rapidly, while for others, it takes as long as 15 years to drop to that of a nonsmoker.

For example, smokers are twice as likely to die of heart disease as nonsmokers, but about half of this excess risk disappears within the first year after quitting.

The reduction in cancer risk is more gradual. A male smoker is 22 times as likely to die of lung cancer as a man who has never smoked. Ten years after quitting, a man who formerly smoked is still 6 to 11 times more likely to die of lung cancer than is a lifelong nonsmoker.

The report also called smoking the most important modifiable cause of infant mortality and premature births in the United States. Smoking during pregnancy slows the growth of the fetus and increases the chances of premature delivery, bleeding and other complications. The report said that, despite these well-recognized risks, about 25 percent of pregnant women in the United States smoke throughout pregnancy.

It concluded that if all women could be persuaded not to smoke during pregnancy, about 5 percent of infant deaths and 20 percent of low-birthweight births could be prevented. It said that even if a woman quits smoking as late as the third or fourth month of pregnancy, her risk of having a low-birthweight baby becomes the same as that of a woman who never smoked.

The report said the weight often gained by smokers who quit poses far less risk to health than continuing to smoke. It said that people who quit smoking gain an average of five pounds and that less than 4 percent of ex-smokers gain more than 20 pounds.

STROKE: Risk 5 to 15 years after quitting is reduced to that of those who never smoked.

ANCERS of the mouth, throat and esophagus: Risk 5 years after quitting is half that of those who still smoke.

CANCER of the larynx: Risk is lower by an undetermined amount compared to those who still smoke.

CORONARY HEART DISEASE: Risk after 1 year is half that of those who keep smoking. After 15 years risk is equal to that of those who never smoked.

CHRONIC OBSTRUCTIVE PULMONARY DISEASE: Risk after "long-term quitting" is reduced by an undetermined amount compared to those who still smoke.

LUNG CANCER: 10 years after quitting the risk may drop to about half that of those who keep smoking.

PANCREATIC CANCER: Risk reduced by an undetermined amount 10 years after quitting.

ULCER: Risk reduced by an undetermined amount after quitting.

BLADDER CANCER: Within a "few years" of quitting, the risk is half that of those who keep smoking.

PERIPHERAL ARTERY DISEASE: Risk is reduced by an undetermined amount after quitting.

CERVICAL CANCER: Risk "a few years" after quitting is reduced.

LOW BIRTHWEIGHT BABY: Risk is reduced to that of those who never smoked if a woman quits smoking before pregnancy or during the first trimester.

SOURCE: Centers for Disease Control