We evaluated the acute hemodynamic responses to hydralazine during cardiac catheterization in 13 infants with idiopathic dilated cardiomyopathy. Ages ranged from 2 to 13 months (6.7 +/- 4.0 months, mean +/- SD). Each infant had congestive heart failure and angiographic evidence of markedly depressed left ventricular ejection fraction (0.24 +/- 0.11; normal = 0.58-0.78) with left ventricular dilation (left ventricular end-diastolic volume = 349 +/- 125% of normal). Hydralazine (0.5 to 1.0 mg/kg administered intravenously) acutely decreased systemic arteriolar resistance from 21.1 +/- 3.3 to 12.0 +/- 2.7 U/m2 (p less than 0.001). This 41 +/- 14% decrease in systemic resistance was accompanied by a 45 +/- 16% increase in cardiac index (3.24 +/- 0.53 to 4.71 +/- 0.99 L/min/m2; p less than 0.001). Mean arterial blood pressure declined from 70 +/- 8 to 60 +/- 11 mm Hg (p less than 0.001). Hydralazine also increased heart rate (122 +/- 19 to 138 +/- 18 bpm; p less than 0.001), but this increase did not account entirely for the change in cardiac index as evidenced by a rise in stroke volume index (26.9 +/- 4.9 to 34.5 +/- 7.5 ml/beat/m2; p less than 0.001). Pulmonary arteriolar resistance and pulmonary capillary wedge pressure fell slightly in response to hydralazine. Subsequently, oral hydralazine was included in the treatment regimen of 10 infants followed for 3 to 38 months (mean = 15 months). Of these, eight demonstrated sustained clinical improvement. We conclude that hydralazine may be a beneficial adjunct to the management of congestive heart failure in young infants with a dilated cardiomyopathy.