Serum testosterone (T) and dihydrotestosterone (DHT) were measured by radioimmunoassay in 14 patients with unaborted hydatidiform mole and in 16 patients with normal pregnancy of similar gestational age. Serum human chorionic gonadotropin (hCG) was measured by the radioreceptor assay in patients with hydatidiform mole. Serum T ranged from 0.27 to 5.39 ng/ml with a mean +/- SE of 2.21 +/- 0.45 ng/ml in patients with hydatidiform mole mole and from 0.20 to 2.40 ng/ml with a mean +/- SE of 0.80 +/- 0.14 ng/ml in patients with normal pregnancy, the difference being statistically significant (P = less than 0.005). Similarly, patients with molar pregnancies had a significantly higher (P = less than 0.005) serum DHT (range: 0.09 to 0.62 ng/ml; mean +/- SE: 0.29 +/- 0.05 ng/ml) than patients with normal pregnancies (range: 0.04 to 0.28 ng/ml; mean +/- SE 0.12 +/- 0.02 ng/ml). There was no significant correlation between uterine size or serum hCG and serum T or DHT. The possible sources of the elevated serum T and DHT and the lack of hirsutism or virilization in patients with trophoblastic disease are discussed.