Serum factor VIII related antigen (factor VIII R:Ag) levels were determined in 34 patients with hydatidiform mole at diagnosis and at 2, 4 and 6 weeks after evacuation of uterus and in 272 normal pregnant women. Serum factor VIII R:Ag levels in molar pregnancies before evacuation of the uterus were significantly higher than those in normal pregnancies of the same gestation age. Serum factor VIII R:Ag levels in the group of patients with residual trophoblastic disease were significantly higher than those in the group without residual trophoblastic disease before evacuation and at 6 weeks after evacuation of uterus, but there is considerable overlap in the levels of factor VIII R:Ag between the two groups of patients especially before evacuation of uterus. Factor VIII R:Ag level does not appear to be an useful marker in predicting the outcome of hydatidiform mole.