We identified 22 women with thrombocytopenia of < 100,000/microliters found incidentally during pregnancy and prospectively monitored their platelet count and clinical outcome for a minimum of 6 months postpartum. During the study period, four women became pregnant twice, accounting for a total of 26 pregnancies. The lowest platelet count during pregnancy was 65,600/microliters +/- 19,400 (mean +/- SD), and at delivery 84,500/microliters +/- 32,300 (P < 0.02). The thrombocytopenia was virtually asymptomatic in all patients during the pregnancy and delivery, whether vaginal or surgical. Neonatal platelet counts (n = 18) were normal (270,700/microliters +/- 69,900), and none of the newborns (n = 24) had a bleeding diathesis. Normalization of the platelet count (i.e., > 150,000/microliters) was documented in 18 patients within 1 month postpartum, in five within 3 months postpartum, and in two as late as 5 months after delivery. One woman did not recover from the thrombocytopenia and eventually developed other stigmata of an autoimmune disease. Long-term follow-up showed recurrence of thrombocytopenia in four patients: three in the context of a subsequent pregnancy and one who developed idiopathic thrombocytopenic purpura. Retrospective analysis of blood counts obtained from 12 previous pregnancies demonstrated thrombocytopenia of a similar degree to the index pregnancy. We conclude that gestational thrombocytopenia of < 100,000/microliters is clinically a benign phenomenon that can recur in subsequent pregnancies and is not accompanied by neonatal thrombocytopenia. In some cases, however, pregnancy-associated thrombocytopenia may be a manifestation of an autoimmune disease with its attendant implications for the neonate. Since the differential diagnosis between the two conditions may be difficult to establish when first encountered during pregnancy, a conservative approach emphasizing careful surveillance and guarded reassurance is justified as long as the platelet counts are > 50,000/microliters.