Coagulation-factor assays were performed on 16 neonates born to 16 unselected epileptic mothers who were being treated with anticonvulsant drugs. 2 of the neonates had clinical evidence of a bleeding tendency. A coagulation defect similar to that in vitamin-K deficiency was found in 8 neonates and was severe in 7. The mothers of the 8 affected neonates had been treated with a barbiturate or with a drug metabolised to a barbiturate and with phenytoin. It is suggested that anticonvulsant drug therapy during pregnancy, especially treatment with barbiturates, frequently causes a coagulation defect which may result in bleeding during the early neonatal period. The prophylactic use of vitamin K1 is especially important in this situation.