We have evaluated the effect of maternal intravenous immunoglobulin G (ivIgG) treatment on platelet counts in fetal alloimmune thrombocytopenia. Seven patients were studied. All of them were multiparous women who had been immunized against the HPA-1a antigen during previous pregnancies and had given birth to at least one severely thrombocytopenic infant. In this study, umbilical blood collection was performed first at the 20th week of gestation and repeated 2-13 times (mean 6 times), depending on the degree of fetal thrombocytopenia. Fetal platelet counting was combined with intrauterine transfusion of 20-30 ml of HPA-1a-negative platelet concentrates to prevent bleeding following umbilical cord puncture. Initial fetal platelet counts ranged from 10,000 to 91,000 per microliters. Maternal treatment with ivIgG (1 g per kg body weight; mean dose 70 g) was given once a week over 7 weeks. In five of seven cases, the basal platelet count did not rise and in two of these cases, it decreased during maternal ivIgG treatment. In one fetus, the baseline platelet count increased from 10,000 to 35,000 per microliters during ivIgG, and in another fetus from 23,000 to 64,000 per microliters. Our observations suggest that ivIgG has no definite benefit for fetal alloimmune thrombocytopenia. Since platelet counts can be very low, careful fetal monitoring by umbilical blood sampling is required. Frequent platelet transfusions in short intervals may be necessary to increase platelet counts in extremely thrombocytopenic fetuses.