Fetal platelet counts were retrospectively studied in a series of 5,194 consecutive fetal blood samplings (FBS). The mean value was 245 +/- 65 x 10(9)/L, without significant variation between 17 and 41 weeks' gestation. After exclusion of false thrombocytopenia due to contamination with amniotic fluid, 247 fetuses had platelet counts less than 150 x 10(9)/L. In 70 cases, thrombocytopenia was due to congenital infectious diseases (toxoplasmosis, rubella, and cytomegalovirus). It was related to immune causes in 45 cases: anti-HPA-1a (n = 23), anti-HPA-5b (n = 2) or possible anti-HLA (n = 2) alloimmunizations, and immune thrombocytopenic purpura (n = 18). Chromosomal abnormality was the etiology in 43 cases (trisomy 13, 18, and 21, Turner's syndrome, triploidy), and other disorders (multiple birth defects, intrauterine growth retardation, rhesus disease, and gestational thrombocytopenia) in 62 cases. No specific cause for the low platelet count could be established in 27 fetuses (range, 115 to 149 x 10(9)/L). Severe thrombocytopenia (< or = 50 x 10(9)/L) occurred mainly in immune cases (16%), congenital infectious diseases (7%), and chromosomal abnormalities (1%). Diagnosis, prognosis, and management of fetal thrombocytopenia are presented in the different clinical situations. In this series, FBS was never associated with serious bleeding, and no fetal exsanguination was observed.