An epidemiologic study was conducted to evaluate the relationship between cigarette smoking and the risk of breast, endometrial, and ovarian cancers. Results indicated a clear reduction of risk for endometrial cancer among women aged 50 years and older who smoke cigarettes. Risks were significantly reduced among moderate smokers (odds ratio = 0.6), heavy smokers (odds ratio = 0.4), and former smokers (odds ratio = 0.6). No association was observed among women under age 50 years. When the relationship between cigarette smoking and breast cancer was investigated, no statistically significant association between cigarette smoking and the risk of breast cancer was observed except among women greater than 50 years who were light smokers only. There was also a nonstatistically significant increase in risk among younger women smoking more than two packs of cigarettes a day (odds ratio = 2.0), but overall there was no evidence of any relationship. Similarly, no association between ovarian cancer and cigarette smoking was apparent, although there was a nonsignificant increase in risk among women under age 50 years who smoked 40 or more cigarettes a day or were exsmokers.