A twenty-five-year follow-up study of women exposed to diethylstilbestrol during pregnancy.

To assess the long-term effects of diethylstilbestrol we conducted a health survey among 693 mothers who had taken the drug during pregnancy and a comparable group of 668 who had not. These women had participated in a study during 1951-52 to evaluate the drug. There were 32 (4.6 per cent) breast cancers among the 693 exposed and 21 (3.1 per cent) among the 668 unexposed, but the difference was not statistically significant (P = 0.16). No statistically significant differences occurred between the groups in any of the other categories of disease. The occurrence of breast cancer in both groups was compared to the Connecticut State Tumor Registry for 1963-65. Compared to the registry data, a significantly (P less than 0.01) higher incidence of breast cancer occurred in both the exposed and unexposed groups at ages over 50. The reason for this increase is not known, but effects linked to the selection of mothers participating in the original clinical study cannot be excluded.