Selenium concentrations in whole blood of Japanese and American women with and without breast cancer and benign fibrocystic breast disease were determined. The observed blood Se levels of healthy Japanese women (0.286±0.021μg/ml) were similar to previously reported values for healthy Japanese adults. The Japanese patients with benign breast disease and with breast cancer exhibited blood selenium concentrations of 0.200±0.045 and 0.195±0.057μg/ml, respectively. The mean blood Se concentration of Japanese breast cancer patients with recurrence was 0.188±0.061μg/ml. The mean blood Se concentrations of healthy American women from San Diego, Calif., were 0.191±0.023μg/ml; of women with benign fibrocystic disease, 0.142±0.010μg/ml; and of breast cancer patients, 0.167±0.032μg/ml. The higher blood Se concentrations of Japanese healthy subjects as compared to healthy Americans can be attributed to differences in the dietary Se intakes; low blood Se concentration may be indicative of increased breast cancer risk.