During pregnancy some women develop unexplained thrombocytopenia (gestational thrombocytopenia). Previous studies have detected abnormal platelet antibodies, suggesting an autoimmune aetiology. To determine whether gestational thrombocytopenia is associated with increased maternal bleeding or adversely affects the fetus, 31 pregnant women with asymptomatic thrombocytopenia were compared with 12 women with thrombocytopenia associated with pre-eclampsia and 34 normal pregnant controls. There was no increase in maternal bleeding in those with asymptomatic thrombocytopenia compared with the normal controls, but pre-eclamptic women experienced more bleeding (mean difference 181 ml, 95 per cent confidence limits 50-312 ml, p < 0.01). There was no difference in the mean weights of the babies or placenta, nor in the APGAR scores between infants born to controls and those with asymptomatic thrombocytopenia. Cord blood platelet levels were measured in 26 women with asymptomatic thrombocytopenia and were normal in 25 and mildly reduced in one. Thus measures used for the treatment and delivery of pregnancies complicated by autoimmune thrombocytopenia are not indicated in gestational thrombocytopenia. Pregnant women should not be considered thrombocytopenic unless the platelet count has fallen below 120 x 10(9)/l.