Breast cancer in mothers given diethylstilbestrol in pregnancy.

We compared the incidence of breast cancer in 3033 women who had taken diethylstilbestrol (DES) in pregnancy during the period from 1940 to 1960 with the incidence in a comparable group of unexposed parous women. We ascertained vital status in 95 per cent of the exposed women and in 93 per cent of the unexposed women and received completed questionnaires for 88 and 85 per cent, respectively. With over 85,000 woman-years of follow-up in each group, the incidence of breast cancer per 100,000 woman-years was 134 in the exposed group and 93 in the unexposed group, yielding a crude relative risk of 1.4 (95 per cent confidence interval, 1.1 to 1.9). The elevated incidence did not appear to be due to bias or to confounding by other risk factors measured in the study. Breast-cancer mortality was slightly higher in the exposed women (relative risk, 1.1) but not significantly so (95 per cent confidence interval, 0.7 to 2.0). We conclude that the incidence of breast cancer is moderately increased in women given DES, but we cannot exclude the possibility that some unrecognized concomitant of DES exposure accounts for this increase.