A histologic study was conducted of sagittal sections of the genital tracts of 281 autopsied female stillborns and neonates. The prevalence of vaginal adenosis among 43 offspring exposed in utero to diethylstilbestrol (DES) was 70%, a frequency 18 times greater than the 4% prevalence among 159 unexposed offspring. The relationship of the prevalence of vaginal adenosis to the gestational age at initial exposure was highly significant: 81% of those first exposed during the period of vaginogenesis had adenosis, whereas none exposed after 21 weeks' gestation had adenosis (P1= 1 ± 10−4). The relationship of the prevalence of vaginal adenosis to the total dose of DES prior to 22 weeks' gestation also was significant (P1= 0.02), and this relationship was independent of gestational age at first exposure (P1= 0.01). In contrast, the prevalence of adenosis among 23 offspring exposed to steroidal estrogens and progestins was about the same as that among the unexposed offspring. Vaginal adenosis was unrelated to the complications of pregnancy for which the hormones were given, the calendar year of birth, and the gestational age at delivery.