Serum human chorionic somatomammotropin (hCS) was measured in 35 patients with intact unaborted hydatidiform mole by a rapid radioimmunoassay using 70% dioxane in water to separate the bound from unbound Traction. Serum hCG was measured by a hemagglulination inhibition method. Serum hCS ranged from 250 to 5900 ng/ml, while serum hCG ranged from 60 IU/ml on unaborted molar pregnancies. Serum hCS in hydatidiform mole increases from a mean ± SE of 650 ± 88.5 ng/ml at 7 to 9 weeks' gestation to 1986.7 ± 859.3 ng/ml at 22 to 25 weeks' gestation. There was a significant correlation between uterine size and serum hCS in molar pregnancies (correlation coefficient r = +0.5183; P = 0.0025). There was no significant correlation between serum hCS and serum hCG. Serum hCS in a patient with a molar pregnancy who subsequently developed choriocarcinoma was not significantly different from that in patients who did not. The findings indicate: 1) that peripheral hCS increases with increased gestational age in molar pregnancies, 2) that the amount of peripheral serum hCS is related to the mass of molar tissue present and not dependent on serum hCG level, and 3) that the serum hCS level in unaborted hydatidiform mole was a poor index for predicting malignant sequelae.