Hormone replacement therapy and endometrial cancer in Ontario, Canada.

Estrogen therapy reduces the risk of osteoporosis and cardiovascular diseases but is associated with an increased risk of endometrial cancer. We have assessed the impact of a regimen of estrogen with progestogen on risk of endometrial cancer for women 48 years and older. We conducted a case-control study in Ontario, Canada, from 1994 to 1998 by interviewing registry-based cases (n = 512) and population controls (n = 513) to obtain information on use of hormones and dietary habits. Compared to non-users, the use of opposed hormone therapy in sequential regimen for more than three years showed a borderline increase in risk (OR = 1.49, 95% CI 0.93-2. 40), but this increase was much less than among women on unopposed estrogen (OR = 4.12, 95% CI 2.21-7.71). Stronger associations were observed when duration of sequential hormone use was examined as a continuous variable (OR per three years of use = 1.21, 95% CI 1.03-1. 42). The effect of opposed hormone therapy on endometrial cancer risk appears to vary both by usage patterns and by patient characteristics of body weight and history of diabetes.