In 1060 Japanese patients, 58 and 36% of breast tumors contained estrogen (ER) and progesterone receptors (PR), respectively. These rates are somewhat lower than those in Western countries. The slightly lower incidence of receptors in Japanese patients is due to significantly lower incidence of receptor positive tumors in postmenopausal Japanese patients than in postmenopausal Western patients. However, in postmenopausal overweight (greater than or equal to 60 kg) Japanese patients the rate of ER + breast cancer was comparable to that in postmenopausal Western patients. The effect of overweight on an increased incidence of Japanese breast cancer was found to be confined to postmenopausal women. In 260 Japanese breast cancer patients, the regression rates to endocrine therapy were 10% for ER-tumors, 48% for ER + tumors, and 66% for ER +, PR + tumors, which are similar to those reported in Western countries.