There have been inconsistent results on the association of postmenopausal obesity with breast cancer risk according to the estrogen (ER) and/or progesterone receptor (PR) status in the breast tissue, and this requires further evaluation. This study was designed to assess whether postmenopausal obesity differs according to receptor status. Information on risk factors was obtained from 1154 breast cancer cases and 21714 controls at Aichi Cancer Center Hospital, Nagoya, Japan between 1988 and 1992. The receptor status was known for 40% of cases. Obese postmenopausal women showed an increased risk of breast cancer (odds ratio (OR) for 5 kg of current weight=1.17, 95% confidence interval (CI)=1.10-1.25; OR for 1 kg/m(2) of body mass index (BMI)=1.07, 95% CI=1.04-1.10). The elevated OR was strongest for ER-positive, as well as with PR-positive, breast cancer among postmenopausal women who had a high BMI. The risk did not differ significantly according to ER status. However, obesity indices among postmenopausal women differed with borderline significance according to PR status. These results are consistent with the hypothesis that there is a gradient of risk for postmenopausal obesity according to hormonal receptor status, at least for PR status, although this was not statistically significant.