In a case-control study, we evaluated the effects of maternal ingestion of acetylsalicylic acid (aspirin) within 10 days of delivery on maternal and neonatal hemostasis. Only one of 34 control maternal-neonatal pairs (3 per cent) had hemostatic abnormalities. In 10 pairs, when maternal aspirin ingestion occurred within five days of delivery, 6 of 10 mothers and 9 of the 10 infants had bleeding tendencies. Seven maternal-neonatal pairs in which aspirin was ingested 6 to 10 days before delivery were free of clinical bleeding. Among seven other mothers who ingested aspirin in the immediate post-partum period four of the seven (57 per cent) also had impaired hemostasis. Neonatal hemostatic abnormalities included numerous petechiae over the presenting part, hematuria, a cephalhematoma, subconjunctival hemorrhage, and bleeding from a circumcision. Maternal bleeding was confined to excessive intrapartum or post-partum blood loss. We conclude that aspirin should be avoided during pregnancy. If ingestion has occurred within five days of delivery, the neonate should be evaluated for the presence of bleeding.