The aim of this study was to determine the associations between hysterectomy, bilateral salpingo-oophorectomy (BSO), and incidence of diabetes in postmenopausal women participating in the Women's Health Initiative (WHI), a series of trials conducted in the United States, during the period 1993-1998. A total of 67,130 postmenopausal women aged 50-79 years were followed for a mean of 13.4 years. Among them, 7,430 cases of diabetes were diagnosed. Multivariable Cox proportional hazards models were used to assess the association between hysterectomy/oophorectomy status and diabetes incidence. Compared with women without hysterectomy, women with hysterectomy had a significantly higher risk of diabetes (hazard ratio = 1.13, 95% confidence interval: 1.06, 1.21). The increased risk of diabetes was similar for women with hysterectomy only and for women with hysterectomy with concomitant BSO. Compared with hysterectomy alone, hysterectomy with BSO was not associated with additional risk of diabetes after stratification by age at hysterectomy and hormone therapy status. In our large, prospective study, we observed that hysterectomy, regardless of oophorectomy status, was associated with increased risk of diabetes among postmenopausal women. However, our data did not support the hypothesis that early loss of ovarian estrogens is a risk factor for type 2 diabetes. The modest increased risk of diabetes associated with hysterectomy may be due to residual confounding, such as the reasons for hysterectomy.