Diabetes-associated endocrine factors may influence the growth of some breast tumors. Therefore, one might expect an association of family history of diabetes mellitus with breast cancer incidence. We analyzed data from a cohort study of 41,837 women ages 55-69 years. During 10 years of follow-up, we identified 1,013 incident cases of breast cancer. There was no consistent association between family history of diabetes mellitus and family history of breast cancer, except among sisters. Women with a family history of diabetes had a higher prevalence of breast cancer risk factors, but family history of diabetes was not associated with risk of postmenopausal breast cancer, regardless of family history of breast cancer status.