OBJECTIVE The authors compared the effects of remifentanil with fentanyl on the hemodynamic and respiratory variables in children with left-to-right shunting and pulmonary hypertension. METHODS A prospective, randomized, and controlled design. METHODS University hospital. METHODS Children aged between 3 months and 6 years undergoing pediatric cardiac surgery for correction of left-to-right intracardiac shunting. METHODS Children were assigned to 1 of the 2 opioids for intraoperative use. Fentanyl was given as a 20 microg/kg intravenous bolus followed by infusion at a rate of 20 microg/kg/h in group 1 (control, n=15), and remifentanil was given as a 2 microg/kg intravenous bolus followed by infusion at a rate of 2 microg/kg/min in group 2 (n=18). RESULTS Mean arterial pressures at 30 to 40 minutes postbypass and the first 2 hours postsurgery were higher in the remifentanil group (p<0.05). Heart rates, pulmonary artery pressures, and airway pressures did not differ at any time between groups. Peripheral oxygen saturation values were lower at 30 and 45 minutes in the prebypass period and higher at 1 to 4 hours in the intensive care unit in the remifentanil group (p<0.05). After protamine administration, transient peripheral oxygen desaturation was observed with 10 children in the remifentanil group and with 3 children in the fentanyl group without any hemodynamic deterioration (p=0.029). CONCLUSIONS There were no clinically important differences in hemodynamic and respiratory measurements intraoperatively and during the initial 24 hours postoperatively between fentanyl and remifentanil in pediatric patients undergoing surgical repair of defects with left-to-right shunts.