The importance of the major risk factors for endometrial cancer in women of different ages was evaluated with the use of data from a hospital-based case-control study conducted in Milan, Italy, on 283 women with endometrial cancer and 566 age-matched controls. Current weight was related strongly to the risk of endometrial cancer both in younger (premenopausal) and in older women (with risk estimates for the heaviest categories of 20.3 and 7.7, respectively), thus confirming that obesity is the major cause of endometrial cancer in Northern Italy. Endometrial cancer risk appeared to be approximately proportional to the second power of body mass index. Early menarche and nulliparity were associated with an increased risk of endometrial cancer in premenopausal women, the point estimate for nulliparity rising to 35.1 (with lower confidence limit of 10.2) after adjustment for marital status. However, no association with these factors was evident in postmenopausal women. Combination oral contraceptives were used by 2 cases and 19 controls only [relative risk (RR) = 0.2, with 95% confidence interval = 0.1-0.8]. The use of noncontraceptive estrogens was associated with an elevated risk, which was greater in perimenopausal women (RR = 5.1 for greater than 2 yr of use), and decreased progressively with increasing time after menopause. Late menopause was also related to endometrial cancer. However, the risk estimates for late menopause apparently were more elevated in older women (greater than or equal to 65 yr) than in perimenopausal women. Most of the risk factors identified (excluding late menopause) apparently act on one of the later stages of the process of carcinogenesis, because the excess risk drops after cessation of exposure.