Circulating immune complex levels were measured in patients with molar pregnancy to investigate the relationship between circulating immune complex and trophoblastic tumor burden. When 27 (87%) of 31 patients with molar pregnancy were first seen, circulating immune complex values were in the normal range. Three of the 4 patients with elevated levels had concurrent medical illness. Eighteen patients were followed with serial measurements until gonadotropin remission was achieved and all 18 patients developed increased levels as they entered remission (P less than .001). Circulating immune complex values remained elevated during gonadotropin remission from 6 to 16 weeks and then declined to initial levels. Further investigation should be undertaken to evaluate possible interactions between circulating immune complex and host immune defenses.