Genetically determined prolongation of the QT interval on ECGs has been proposed as one basic pathogenetic mechanism for the sudden infant death syndrome (SIDS). ECG studies in a total of 108 first-degree relatives of 26 patients with this syndrome in comparison with 99 such subjects from 22 control families failed to show any significant differences in the QT interval in these two groups. Hereditary prolongation of the QT interval is therefore unlikely to be a significant factor in the etiology of the vast majority of cases of SIDS.