BACKGROUND. Endometrial carcinoma and endometrial hyperplasia share a number of risk factors, particularly exposure to oestrogen stimulation. Studies have shown that cigarette smoking protects against carcinoma. This study was undertaken to confirm this finding in carcinoma, and to extend the investigation to hyperplasia. METHODS. Data from a previously published population-based case-control study, conducted in Toronto, Ontario were used to model risk estimates. RESULTS. Smoking modestly reduced the risk, (although not statistically significantly), for hyperplasia among pre- and postmenopausal women, and for carcinoma among postmenopausal women. In contrast, smoking appeared to elevate risk for carcinoma among premenopausal women, although the number of cases upon which these estimates are based are very small. CONCLUSIONS. Among the shared risk factors for endometrial hyperplasia and endometrial carcinoma is cigarette smoking. As these results are consistent with other studies, it seems apparent that the mechanism of action for cigarette smoking may be different in premenopausal women. Hyperplasia could provide a useful model for studying the smoking effect on endometrial neoplasms in premenopausal women.