Risk factors for breast cancer in a cohort of women who participated in the first National Health and Nutrition Examination Survey (NHANES) and its followup epidemiologic survey were examined. The analytic cohort consisted of 122 breast cancer cases and 7,304 noncases, with a median followup time of 10 years. We found no appreciable increase in risk among women who reported their onset of menarche as occurring before the age of 13 compared with those reporting onset at ages 13 and older. Breast cancer risk was progressively elevated with increasing age at first live birth (test for trend, P less than 0.007). The number of children born to a woman did not influence risk, but the data suggested an increased risk for nulliparous women. A family history of breast cancer in a first-degree relative was the strongest predictor of risk for this cohort of women, with relative risks of 2.2 and 2.4 associated with a mother or sister affected with breast cancer, compared with women having no family history. The age of natural menopause had little influence on breast cancer risk, and the data suggested a slight protective effect of early surgical menopause. Higher education (compared with less than a high school education) was associated with an increased risk in this cohort of women (relative risk (RR) = 2.1; 95 percent confidence interval (CI) = 0.9-5.1). These results (a) confirm the importance of some well-recognized risk factors for breast cancer in a cohort of women, followed prospectively for 10 years, and perhaps more importantly, (b) uniquely provide risk estimates on a probability sample of women in the United States.