A retrospective study of all inborn infants at 26 to 35 weeks' gestational age delivered from August, 1976, through July, 1977, was undertaken to determine the effects on the neonate of maternal isoxsuprine therapy for premature labor. Mothers of 43 infants received ISX within 48 hours of delivery and mothers of 107 received no ISX. Hypocalcemia, hypoglycemia, evidence of ileus, hypotension, and neonatal death were all significantly more common in infants whose mothers received ISX. Hypotension and death occurred predominantly in infants of 26 to 31 weeks' gestation and in infants whose mothers developed hypotension or tachycardia during ISX infusion. The frequency of hypotension and death decreased as the time interval from the loading dose of ISX to delivery increased.