OBJECTIVE: Using data from a case-control study of endometrial cancer, we investigated the relationship between the progestin and estrogen potency in combination oral contraceptives (OCs) and the risk of developing endometrial cancer. METHODS: Subjects included 434 endometrial cancer cases and 2557 controls identified from the Cancer and Steroid Hormone (CASH) study. OCs were classified into four categories according to the individual potencies of each hormonal constituent (high versus low estrogen or progestin potency). Logistic regression was used to evaluate associations between endometrial cancer risk and combination OC formulations. RESULTS: With non-users as the referent group, use of OCs with either high potency progestin [odds ratio for endometrial cancer (OR) = 0.21, 95% confidence interval (CI) = 0.10 to 0.43] or with low potency progestin (OR = 0.39, 95% CI = 0.25 to 0.60) were both associated with a decreased risk of endometrial cancer. Overall high progestin potency OCs did not confer significantly more protection than low progestin potency OCs (OR = 0.52, 95% CI = 0.24 to 1.14). However, among women with a body mass index of 22.1 kg/m2 or higher, those who used high progestin potency oral contraceptives had a lower risk of endometrial cancer than those who used low progestin potency oral contraceptives (OR = 0.31, 95% CI = 0.11 to 0.92) while those with a BMI below 22.1 kg/m2 did not (OR = 1.36, 95% CI = 0.39 to 4.70). CONCLUSION: The potency of the progestin in most OCs appears adequate to provide a protective effect against endometrial cancer. Higher progestin-potency OCs may be more protective than lower progestin potency OCs among women with a larger body habitus.