The number of American men and women who have quit smoking cigarettes has increased sharply over the past two decades, according to preliminary new data reported today by the American Cancer Socierty.

Among men surveyed last year, 38.7 percent were ex-smokers--more than double the 17.3 percent in a 1959 ACS study. Among women, 21.1 percent were former smokers, a nearly four-fold increase over the 5.6 percent in 1959.

ACS scientists Steven D. Stellman and Lawrence Garfinkel emphasized that the men and women in the study who gave up cigarettes are older and better educated than the American population as a whole. It would be "very chancy" to project the results to the general population, Stellman told reporters.

The researchers disclosed the preliminary results here at a session of the Fifth World Conference on Smoking and Health.

The new data are from the largest epidemiological survey of its kind--a prospective study of death rates among approximately 1.2 million men and women in all 50 states, the District of Columbia and Puerto Rico.

The preliminary analysis reported today covered 13 percent (157,936) of the participants in "Cancer Prevention Study II," which is at least six years from completion.

Like CPS I, which was begun in 1959 and completed in 1972, the new survey is intended "to determine death rates from cancer and other causes among the study population, in relation to various environmental and life-style factors."

The factors include newer types of cigarettes, particularly those lower in tar and nicotine; various foods; vitamins and medications; artificial sweeteners; birth control pills; work-place conditions, and some kinds of stress.

The analysis announced today involves persons who are mostly over 45, although a few are in the 40-to-44 bracket. Of the men, 32 percent are college graduates and of this group half have some post-graduate education.

Other highlights from the preliminary report:

* The proportion of women who have never smoked fell nearly 10 percentage points, from 67.2 percent in 1959 to 57.5 percent in 1982; by contrast, the proportion of men who have never smoked increased by 2.8 percentage points, to 25.5 percent.

* The share of current smokers among both sexes declined, from 48.4 percent to 27.7 percent among men; the decline among women was much smaller, from 27.2 percent to 21.4 percent.

* Male pipe and cigar smokers decreased by 3.5 percentage points, to 8.1 percent.

The ACS began CPS II last fall by enlisting 78,000 volunteer "researchers" to distribute the questionnaires to persons they knew well and could follow for several years. Follow-ups are planned in 1984, 1986 and 1988.

In another paper, researchers William Pollin and R.T. Ravenholt of the National Institute on Drug Abuse said that 56 million Americans smoke and that smoking kills 350,000 of them annually.

"After 25 years of decline, 1982 saw an unfortunate upturn in the percentage of Americans smoking," they reported, citing Gallup Poll data. "Among American adolescents," they said, there was "a reversal of eight years of growth in rejection and perceived health risks associated with smoking."

Among high school seniors, the upturn in smoking of a half-pack or more a day was slightly higher among females than males.