OBJECTIVE To review our experience with, and to evaluate the efficacy of, antenatal pharmacologic treatment of pregnancies complicated by alloimmune thrombocytopenia. METHODS We reviewed the records of six pregnancies complicated by alloimmune thrombocytopenia recently cared for at the University of Iowa Fetal Diagnosis and Treatment Unit. All patients had a history consistent with alloimmune thrombocytopenia in a previous gestation. All fetuses had thrombocytopenia on funipuncture at 20-32 weeks' gestation, and all patients and fetuses demonstrated a platelet antigen incompatibility. Three women initially received weekly gamma globulin infusions, two received gamma globulin and dexamethasone, and one had no initial treatment but was given gamma globulin and dexamethasone at 32 weeks' gestation. Repeat funipuncture was performed at 3.5- to 7-week intervals, and therapeutic modifications were made as necessary. RESULTS In five cases, the last funipuncture before delivery documented platelet counts adequate for vaginal delivery. One woman, who received gamma globulin alone with good initial response, was delivered by cesarean for a platelet count of 25,000/microL at 39 weeks. Following delivery, all infants were thoroughly evaluated, and none had evidence of intracranial hemorrhage or other alloimmune thrombocytopenia-associated morbidity. All had normal platelet counts at discharge from the hospital. CONCLUSIONS Our experience confirms the efficacy of gamma globulin treatment, but indicates that not all fetuses will respond to it alone. Serial funipunctures are essential to evaluate patient response and allow appropriate therapeutic modifications. Randomized studies are needed to determine the optimal antenatal pharmacologic therapy for this disease.