For a population-based, case-control study of cancer of the endometrium in Greater Boston from 1965 through mid-1969, 440 cases were drawn from nearly all hospitals in the area; controls were drawn at random from the general population. The age-adjusted incidence rate was 18.1/100,000 woman-years, with a peak at ages 55-59 and a gradual decline thereafter. Information was provided from 212 cases and 1,198 controls by mall questionnaire. A trend of reduced risk of endometrial cancer with increased parity was noted, the relative incidence (RI) for multiparous women being 0.3 compared to a RI of unity for married nulliparous women. The association of risk with age at first birth was irregular. Early menarche (RI=1.6) and late menopause (RI=1.7) were associated with increased risk of disease. Endometrial cancer risk was also found to be directly related to socioeconomic status, relative weight, diabetes, hypertension, and arthritis. The findings supported the idea that hormone activity during, and perhaps after, reproductive life is an important cause of this disease.