Myocardial necrosis and fibrosis is a rare complication of cystic fibrosis (CF) causing sudden and unexpected death in infancy due to cardiac arrest. Characteristic morphological lesions are recognisable postmortem. The 18 CF patients with this complication had varied clinical features including mild pulmonary involvement, early onset severe pancreatic insufficiency, and profound electrocardiogram (ECG) changes. In this group of patients, 5 were deltaF508 homozygotes, 1 was deltaF508/ N1303K and 1 was a deltaF508/M compound heterozygote. A pair of affected siblings (deltaF508 homozygotes) were fully concordant for myocardial involvement and for the general course of the disease. The co-existence of a genetic predisposition to myocardial lesions resulting most probably from severe cystic fibrosis transmembrane (CFTR) genotypes (such as deltaF508/deltaF508, deltaF508/N1303K) and deficiency of certain trophic factors necessary for metabolism of the myocardium, are postulated to cause myocardial complications in CF leading to circulatory failure and early death.