The relationship between the strength of the estrogenic and progestational components of combined oral contraceptives and risk of endometrial carcinoma was examined in a multinational hospital-based case-control study comparing 220 cases from 7 countries with 1,537 age- and hospital-matched controls. Oral contraceptives were classified according to their relative content of estrogen and progestin. The risk was not altered in women who used preparations containing high-dose estrogen/low-dose progestin compounds [odds ratio (OR) = 1.10, 95% CI = 0.13-9.96]. In contrast, the risk among users of low-dose estrogen/high-dose progestin oral contraceptives was greatly decreased (OR = 0, 95% CI = 0-1.08). Risks observed for users of high-dose estrogen/high-dose progestin (OR = 0.15, 95% CI = 0.045-0.50) and low-dose estrogen/low-dose progesterone (OR = 0.59, 95% CI = 0.26-1.30) preparations were intermediate between those for users of the 2 other types of preparation. A significantly lower risk was observed for high-dose progestin users than for low-dose users [ratio of odds ratios (ROR) = 0.21, 95% CI = 0.05-0.84]. Although the numbers are small, findings from this study suggest that combined oral contraceptives with varying strengths of estrogen and progestin have different effects on the risk of endometrial carcinoma.