Gestational diabetes is becoming increasingly common; it is important to determine how it relates to future risk of disease. We investigated the relation of gestational diabetes to breast cancer in 37,926 women who had one or more live births in 1964-1976 for whom information had been collected on complications of pregnancy. In this cohort there were 1,626 cases of breast cancer reported to the Israel Cancer Registry before January 1, 2005 and 410 cases of gestational diabetes recorded from birth records. There were 29 cases of breast cancer among women diagnosed with gestational diabetes. Using Cox proportional hazards models to control for age and birth order at the first observed birth and other characteristics, we found that the incidence of breast cancer was increased among women diagnosed with gestational diabetes (relative rate = 1.5, 95% confidence interval 1.0-2.1). This effect was seen only among women 50 years and older (relative rate 1.7, 95% confidence interval 1.1-2.5) but not among women <50 (relative rate = 1.0, 95% confidence interval 0.5-2.1). The findings suggest that gestational diabetes may be an important early marker of breast cancer risk among post-menopausal women, but these results need to be confirmed in future studies.