Relations between platelet counts and mean pulmonary arterial pressures, capillary oxygen tension, and haematocrit were studied in 37 patients, aged 2 to 16 months, with simple aortopulmonary transposition (an intact ventricular septum and no ductus arteriosus). There was a statistically significant inverse relation between pulmonary arterial pressures and platelet counts. Six out of the eight patients (4-16 months old) with raised mean pulmonary arterial pressure had thrombocytopenia (less than 100,000/mm3) and none of the 25 patients with a normal mean pulmonary pressure (less than or equal to 20 mm Hg) showed thrombocytopenia. Pulmonary arterial pressures were not measured in four patients: the platelet count was 7000/mm3 in the patient with a left ventricular systolic pressure of 90 mm Hg but it was greater than 100,000/mm3 in the three in whom left ventricular systolic pressure was less than 50 mm Hg. Platelet counts were positively correlated with capillary oxygen tension and inversely correlated with haematocrit in all 37 patients. Despite the persistence of pulmonary hypertension, thrombocytopenia improved after corrective surgery in five patients, even in those who had thrombocytopenia before operation. These data suggest that the presence of thrombocytopenia is not solely related to the development of pulmonary vascular disease. Pulmonary vascular disease, however, does contribute to the early development of thrombocytopenia in patients with simple aortopulmonary transposition.