Serial serum progesterone and beta human chorionic gonadotropin (beta hCG) levels were measured during surveillance of 24 women at risk for development of gestational trophoblastic neoplasia (GTN) following evacuation of complete molar gestations. Six of the 24 patients developed post-molar GTN. The initial median progesterone level of 76 ng/ml in these six patients drawn at evacuation was significantly higher than the median of 18 ng/ml in those not developing GTN (P = 0.026). Additionally, the serum progesterone decreased to < 5 ng/ml within a week of evacuation in 16/18 patients without GTN. In 5/6 GTN cases, levels of progesterone remained > 5 ng/ml for > or = 3 weeks of the surveillance period (P < 0.05). Serum beta hCG levels required 4-11 weeks of surveillance to distinguish between non-persistent cases and GTN. We conclude that serial serum progesterone levels measured during post-molar surveillance parallel beta hCG regression.