Why is diabetes mellitus a stronger risk factor for fatal ischemic heart disease in women than in men? The Rancho Bernardo Study [published erratum appears in JAMA 1991 Jun 26;265(24):3249] We report here the 14-year sex-specific effect of non-insulin-dependent diabetes mellitus on the risk of fatal ischemic heart disease in a geographically defined population of men and women aged 40 through 79 years. There were 207 men and 127 women who had diabetes at baseline based on medical history or fasting hyperglycemia. They were compared with 2137 adults who had fasting euglycemia and a negative personal and family history of diabetes. The relative hazard of ischemic heart disease death in diabetics vs nondiabetics was 1.8 in men and 3.3 in women, after adjusting for age, and 1.9 and 3.3, respectively, after adjusting for age, systolic blood pressure, cholesterol, body mass index, and cigarette smoking using the Cox regression model. The sex difference in the independent contribution of diabetes to fatal heart disease was largely explained by the persistently more favorable survival rate of women (than men) without diabetes. 