Thirty-three of 616 consecutively admitted newborn infants had trombocytopenia (platelet count less than 150,00/mm). Only 16 of these were among the 356 infants with lung disease. However, 12 of the 16 were among the 90 infants with a diagnosis of a perinatal aspiration syndrome. The 12 thrombocytopenic infants were the only infants with PAS considered to have pulmonary hypertension. The duration of significant right-to-left shunting of blood paralleled the duration of thrombocytopenia in these infants; PHN was not associated with thrombocytopenia in other neonatal lung diseases. Thus, platelets appear to be important in the pathogenesis of PHN complicating PAS.