We report a 10-month-old male infant who was admitted to our hospital with a history of failure to thrive and bulky stools. On examination, he was dystrophic and had a protruding abdomen, but he was well oxygenated and his lungs were clear on auscultation. A tachycardia of 145 beats per min and radiological evidence of cardiomegaly indicated involvement of the heart, but an ECG failed to show signs of myocarditis or cardiac hypertrophy. An elevated sweat chloride concentration of 141 mEq/l confirmed the diagnosis of cystic fibrosis (CF). Molecular analysis revealed heterozygosity for the common mutation delta F508. He died unexpectedly of a sudden cardiac arrest 2 days later. Autopsy revealed scattered myocardial necrosis and fibrosis. Some 50 documented cases of myocardial fibrosis in infants with CF have been reported. Suggested causes such as malnourishment and hypovitaminosis remain speculative as systematic studies have yet to be done.