We have studied retrospectively 18 infants who have received captopril for treatment of severe heart failure due to left-to-right shunts with pulmonary hypertension. Captopril has been administered in doses of up to 3.5 mg/kg/day (mean 2.47 mg/kg/day). Maintenance treatment with digoxin and frusemide was continued but potassium-sparing diuretics were stopped in most patients. The mean period of assessment was 19 days before and 27 days after commencing captopril. The mean daily weight gain before captopril was -7 g and after its introduction was + 13 g (P less than 0.001). There were statistically significant (P less than 0.05) falls in mean heart rate and respiratory rate and rises in plasma sodium concentration and feeding score. Plasma urea concentration fell but this did not reach statistical significance. Two patients suffered hypotension after increments in captopril dosage and subsequently had a rise in plasma urea and creatinine values. This adverse reaction may be linked to the presence of hyponatraemia. This preliminary report shows captopril may be useful in the control of severe heart failure in infancy.