Because the role hormonal and reproductive factors play in the etiology of endometrial cancer is incompletely understood, the authors evaluated the risk of this cancer in relation to age at last delivery. The data were obtained in a hospital-based case-control study of 483 women with endometrial cancer (cases) and 693 women with other conditions (controls) conducted in a number of hospitals in the United States and Canada from 1978 to 1985. There was an inverse association between age at last delivery and endometrial cancer risk: Compared with women who last gave birth before age 25 years, the rate ratio was 0.9 (95% confidence interval (CI) 0.5-1.6) for last delivery at ages 25-29 years, 0.6 (95% CI 0.3-1.0) for last delivery at ages 30-34 years, 0.5 (95% CI 0.3-1.0) for last delivery at ages 35-39 years, and 0.4 (95% CI 0.1-0.9) for last delivery at age 40 years or older. The trend of decreasing rate ratio with increasing age at last delivery was statistically significant (p = 0.02). The association was apparent regardless of parity or menopausal status. There was no evidence for an association between age at first pregnancy and risk. These data suggest that women who bear children late in reproductive life may be at lower risk for endometrial cancer than those who complete their families early.