Smokers who quit will improve their chances for life by more than half -- no matter how old they are or how long they have smoked.

This news comes from a California medical study of nearly 26,000 patients for 13 years. It is one of the largest groups in the history of cigarette-smoking research to have been followed so long and analyzed so meticulously.

Some other studies also have concluded that quitting can help even longtime smokers. But some statisticians have been challenging these results, saying those who quit may be "a different population" in many ways from those who do not.

To settle this question, the Oakland, Calif., Kaiser-Permanente Medical Care Program staff did a "multivariate analysis" of 25,917 patients, an analysis that ruled out such other factors as alcohol consumption and weight.

The analysis, published in today's New England Journal of Medicine, showed with what the Kaiser group called a high degree of certainty that:

The main risk of cigarette smoking is an added risk of developing coronary heart disease. Despite all the publicity about the link between smoking and lung cancer, heart disease poses a greater risk than lung cancer, emphysema or other severe lung diseases.

The average risk of dying from coronary disease over the 13-year period from 1964 to 1977 was more than twice as great among presistent smokers than it was among quitters.

The average risk of dying from any cause was 1.6 times greater among presistent smokers than among quitters. Or, to put this statistic differently, the chance of continuing to live was 60 percent greater for quitters. Or, to put this statistic differently, the chance of continuing to live was 60 percent greater for quitters compared with nonquitters in all age groups.

For all causes of death taken together, even including injuries and poisonings, persistent smokers had the highest death rates, temporary or on-again, off-again quitters the second highest, firm quitters the third highest and never-smokers the lowest.

The persistent smokers had the highest death rates for lung cancers combined, not just for lung cancer, and the highest rates for all circulatory disease (all heart and blood vessel disorders, including strokes), not just for coronary heart disease.

Members of the Kaiser research group that made the report were Drs. Gary Friedman, Diana Petitti and Richard Bawol, with A.B. Siegelaub.

"What we found is that the argument that those who stop smoking have lower death rates because they're 'different' just doesn't hold up," said Dr. Petitti. "So we're back to believing there's a causal" -- that is, a real cause-and-effect -- relationship "between smoking and coronary disease."

About one in four men and one in five women smokers succeed in a quitting, according to a recent federally financed survey of those who had tried to quit within the year before the study was conducted.

But the total number of quitters has reached the millions, and scientists think this helps explain Americans' increasingly lower average risk of fatal heart disease.