The aim of this study was to evaluate risk factors for occurrence of neonatal passive immune thrombocytopenia (PIT) in pregnancy complicated by idiopathic thrombocytopenic purpura (ITP). We studied 63 pregnant women with ITP and the 66 neonates retrospectively. Neonatal platelet counts were compared with maternal platelet counts, platelet-associated gamma G immunoglobulin (PAIgG) values, and the presence of antiplatelet antibody in the maternal circulation, history of previous PIT, maternal treatments for ITP, and other maternal or neonatal factors. PIT (platelet counts <100 x 10(3)/microL) was observed in 9 (14.3%) of 63 pregnancies. Presence of circulating antiplatelet antibody in maternal blood, splenectomy prior to pregnancy, and history of previous PIT were observed more frequently with statistical significance in patients giving birth to neonates who developed PIT. No effect on occurrence of PIT was found by the administration of corticosteroids or immunoglobulin. Splenectomy prior to pregnancy was found by logistic regression analysis to be a single significant variable (p = 0.021, odds ratio 7.20, confidence intervals: 1.35 to 38.3) among the risk factors for PIT.