The relationship between body fat distribution, measured by the ratio of waist-to-hip circumferences (WHR), and the 2 year incidence of diabetes mellitus was examined in a cohort of 41,837 women aged 55-69 years. The 399 women who reported the new onset of diabetes had a significantly greater mean body mass index (kg/m2) and WHR than non-cases. After adjustment for body mass index (BMI), age and education level using multivariate logistic regression, WHR was a significant independent predictor of diabetes in a dose-response fashion. Cases were 4.6 times (95% CI = 3.8, 5.6) more likely than non-cases to be in the upper tertile of WHR and 2.2 times (95% CI = 1.8, 2.7) more likely to be in the middle tertile. Women in the highest tertiles of both WHR and BMI had a 14.4-fold (95% CI = 9.5, 21.9) higher risk of diabetes than women in the lowest tertiles. These results demonstrate that increased abdominal adiposity is a significant independent risk factor for the development of diabetes mellitus in older women.