No prospective data exist on the risk of microbiologically confirmed urinary tract infection (UTI) and asymptomatic bacteriuria (AB) in relation to diabetes mellitus and its characteristics. The authors prospectively (1998-2002) followed 218 diabetic and 799 nondiabetic Washington State women aged 55-75 years for UTI and AB. The baseline examination and two annual follow-up examinations included urine culture, measurement of hemoglobin A1c and postvoid residual bladder volume, and a survey of diabetes and other characteristics. Surveillance for UTI included self-reports confirmed by microbiologic culture and medical record review. UTI incidence per 100 person-years was 12.2 for diabetic women and 6.7 for nondiabetic women (relative risk (RR) = 1.8, 95% confidence interval (CI): 1.2, 2.7). AB incidence per 100 person-years was 6.7 for diabetic women and 3.0 for nondiabetic women (RR = 2.3, 95% CI: 1.3, 3.9). In Cox models adjusted for multiple covariates, the increased UTI risk occurred mainly in women taking insulin (RR = 3.7, 95% CI: 1.8, 7.3) and women with a longer diabetes duration (> or =10 years; RR = 2.6, 95% CI: 1.3, 5.1) compared with nondiabetic women. No clear linear trend between hemoglobin A1c and UTI or AB risk was seen. Postmenopausal women with diabetes have higher risks of UTI and AB in relation to diabetes duration and severity but not to recent glucose control.