Interferon-alpha (IFN-alpha) is a biological response modifier with antiviral and tumoral effect that is used in the treatment of chronic myelogenous leukemias. Adverse effects are well documented and cardiovascular disturbances mostly include hypotension and tachycardia and rarely cardiomyopathy. We report on an infant with chronic myelomonocytic leukemia (CML) diagnosed at 3 months of age who was treated with increasing IFN-alpha dosage (2.5-5.5 million U/m2/day) given subcutaneously for 7.5 months. At that age, he presented anorexia, general malaise, and nocturnal sweating for about a week, followed by respiratory distress and tachycardia. Diagnosis of congestive heart failure was suspected and documented by cardiomegaly and echographic changes of left ventricular dilated cardiomyopathy, with a 40% left ventricular ejection fraction (EF) and 20% fractional shortening (FS). He was treated with digoxin, furosemide, and angiotensin converting inhibitors, and IFN-alpha was discontinued. Progressive improvement of cardiac function was observed within 7 months of the events with normalization of the echocardiographic findings (EF 60%, FS 31%). We should emphasize the possibility of severe and reversible cardiac toxicity of IFN-alpha in infancy.