['To determine retrospectively the effect of high-dose opiate-oxygen (O2) anesthetic technique on intraoperative ventricular fibrillation in high-risk neonates.', 'Retrospective chart review of different anesthetic techniques in a partially contemporaneous patient group (1981 to 1983).', 'Cardiac anesthesia service at a university pediatric hospital.', 'Forty neonates undergoing Norwood Stage I repair of hypoplastic left heart syndrome.', 'High-dose fentanyl-O2 anesthesia in 30 neonates and low-dose morphine sulfate 50%-nitrous oxide (N2O) in 10 neonates.', 'Clinical condition assessed by preoperative and intraoperative arterial blood gases, requirements for sodium bicarbonate (NaHCO3), need for inotropic and pressor support, and vital signs. Outcome assessments by intraoperative ventricular fibrillation (frequency before and after bypass) and hospital mortality. Clinical condition and hospital mortality were no different. The frequency of intraoperative ventricular fibrillation was significantly different: 3% with high-dose fentanyl and 50% with morphine-N2O (p less than 0.005).'] Does high-dose fentanyl reduce intraoperative ventricular fibrillation in neonates with hypoplastic left heart syndrome?