Recent studies suggest that prenatal or early post-natal factors may influence future breast cancer risk. To investigate whether month of birth is a risk factor for breast cancer, we analyzed the distribution of month of birth for 81,162 women died of breast cancer and 1,334,650 women died of cancers at all sites, who were reported to the Ministry of Health and Welfare from 1972-90 in Japan. After considering each birth year seasonal variations in month of birth were not shown statistically significant in any age-group through using chi 2 test (df = 11). The negative results were also confirmed in residential area and period category.