It is unclear whether hypertension and antihypertensive medication use are associated with breast cancer. In order to examine these associations, we conducted a case-control study among women aged 50-75 years. Breast cancer cases were ascertained via a population-based cancer registry (n=523) and controls were ascertained via random-digit-dialing (n=131). Participants completed a self-administered questionnaire which queried history of hypertension, antihypertensive medication use and risk factors. Unconditional logistic regression was used to estimate the odds ratio (OR) and 95% confidence intervals (CI), adjusted for age, body mass index (BMI), diabetes, smoking, alcohol use, menopausal status, family history of breast or ovarian cancer, age at first full-term pregnancy and education. History of treated hypertension was associated with significant increased risk of breast cancer (OR, 1.77; 95% CI, 1.04-3.03) and this association appeared only in women with BMI > or =25 kg/m(2) (OR, 2.30; 95% CI, 1.12-4.71). Diuretic use was also associated with elevated breast cancer risk (OR, 1.79; 95% CI, 1.07-3.01). The risk associated with diuretic use increased with duration of use (P for trend, <0.01). Use of other blood pressure medications was not found to be associated with breast cancer risk. These results support a positive association between treated hypertension, diuretic use and breast cancer risk among women aged 50-75 years.