To study the risk factors associated with breast cancer in women younger than 40 years, a cohort study (The Jerusalem Perinatal Study) of 42 822 female offspring born in hospitals in West Jerusalem during 1964-1976 was carried out. Hazard ratios of potential parental and perinatal risk factors for early breast cancer were measured. The overall incidence of breast cancer was 5.2/100 000 person-years. The highest incidence was found among Jewish women of West Asian ancestry (8.6/100 000 person-years), specifically those whose maternal grandfathers were born in Iraq, Iran or Afghanistan (9.5/100 000 person-years). Using Cox models we found independent risk factors for early breast cancer to be paternal age (relative risk/year=1.06, 95% confidence interval=1.02-1.10, P=0.005), and ancestry from Iraq/Iran/Afghanistan (relative risk=3.1, 95% confidence interval=1.50-6.52, P=0.002). The study confirms a previously observed effect of advanced paternal age on the occurrence of early breast cancer and identifies a novel population group at increased risk for the disease. The excess risk of early breast cancer associated with ancestry from Iraq, Iran and Afghanistan suggests involvement of genetic determinants, environmental exposures and/or lifestyle factors and mandates further investigation.