Alloimmune thrombocytopenia (AIT) is characterized by severe thrombocytopenia, usually diagnosed after birth, which may result in intracranial hemorrhage (ICH) in as many as 20% of cases. The course of AIT typically worsens in subsequent pregnancies. Administration to mother of intravenous gammaglobulin (IVIG) and/or corticosteroids to increase the fetal platelet counts of a subsequent affected fetus is widely used to avoid ICH. The objective of this study was to evaluate the long-term effects of AIT and its antenatal treatment on the medical and developmental outcomes of affected children. Seventy-one pairs of untreated (older) and antenatally treated (younger) siblings with AIT were compared. A medical questionnaire and the Behavioral Assessment System for Children (BASC) were completed over the telephone by mothers. In this sample, birth platelet counts of treated fetuses were significantly higher than those of the untreated fetuses. Treated children were born at significantly lower gestational ages and with significantly lower birthweights than untreated children. No treated child suffered a perinatal ICH compared with 12 untreated siblings. Treated siblings also had fewer vision problems (three versus 14 in the untreated group). Children treated as fetuses received higher scores on the BASC Adaptive Skills Composite than their untreated siblings. The antenatal regimen of IVIG and/or corticosteroids did not affect the results. Children with AIT treated as fetuses had better long-term developmental-behavioral outcomes than their untreated siblings, perhaps because of higher in utero platelet counts. We speculate that platelets may possibly play a role in neurodevelopmental processes in the fetus.