OBJECTIVE: This study aims to determine the positive and negative predictive values of self-reported diabetes during the Women's Health Initiative (WHI) clinical trials. METHODS: All WHI trial participants from four field centers who self-reported diabetes at baseline or during follow-up, as well as a random sample of women who did not self-report diabetes, were identified. Women were surveyed regarding diagnosis and treatment. Medical records were obtained and reviewed for documented treatment with antidiabetes medications or for physician diagnosis of diabetes supported by laboratory measurements of glucose. RESULTS: We identified 1,275 eligible participants; 732 consented and provided survey data. Medical records were obtained for 715 women (prevalent diabetes, 207; incident diabetes, 325; no diabetes, 183). Records confirmed 91.8% (95% CI, 87.0-95.0) of self-reported prevalent diabetes cases and 82.2% (95% CI, 77.5-86.1) of incident diabetes cases. Among those who never self-reported diabetes, there was no medical record or laboratory evidence for diabetes in 94.5% (95% CI, 89.9-97.2). Women with higher body mass index were more likely to accurately self-report incident diabetes. In a subgroup of participants enrolled in fee-for-service Medicare, a claims algorithm correctly classified nearly all diabetes cases and noncases. CONCLUSIONS: Among WHI clinical trial participants, there are high positive predictive values of self-reported prevalent diabetes (91.8%) and incident diabetes (82.2%) and a high negative predictive value (94.5%) when diabetes is not reported. For participants enrolled in fee-for-service Medicare, a claims algorithm has high positive and negative predictive values.