Serum 17alpha-hydroxprogesterone (17-OHP), progesterone (P), and human chorionic gonadotropin (hCG) levels were measured by specific radioimmunoassay in 19 patients undergoing laparoscopy or laparotomy with either unevacuated molar pregnancy or nonmetastatic gestational trophoblastic neoplasms (GTN), in 10 normal pregnant patients at equivalent gestational age (7-21 weeks), and in 4 patients with metastaic GTN following hysterectomy and bilateral salpingo-oophorectomy. All patients with theca lutein cysts had significantly elevated serum 17-OHP levels compared to those in 1) normal pregnancy, 2) patients with GTN and normal-size ovaries, 3) patients with metastatic GTN in the absence of ovaries (P less than 0.02). Levels of serum 17-OHP but not P correlated with the degree of ovarian enlargement (r = 0.87, P less than 0.05). Serum P concentrations in patients with theca lutein cysts, although higher than the levels in cases of GTN with normal-size ovaries, were not significantly different from the levels in normal pregnancy (P greater than 0.05). Serum hCG levels in patients with theca lutein cysts, though higher than the normal pregnancy levels (P less than 0.05), were not significantly different from those in cases of GTN with normal-size ovaries and GTN without ovaries (P greater than 0.05). Under the conditions studied, no correlation was observed between serum hCG and P levels in our cases of GTN. Increased serum 17-OHP level in a patient with GTN suggests the presence of theca lutein cysts.