We have observed in 11 infants, aged 2 years or less, a distinct clinicopathologic lesion responsible for tachyarrhythmias that were fatal in 96% (25/26) of previously reported cases. Nine of the 11 patients, who underwent electrophysiologic mapping and surgical excision of the lesion, have survived, with follow-up periods ranging from 1 month to 6 years. The morphologic findings in these 11 patients and in the 26 cases cited in the literature are reviewed. Pathogenic considerations have included viral-induced lesion, cardiomyopathy, neoplasm, and developmental disorder of Purkinje cells. We believe this lesion to be a myocardial hamartoma. Supportive evidence includes prevalence in infants, tumorlike growth pattern without mitotic figures, and association of other developmental abnormalities. Through electrophysiologic mapping, this myocardial hamartoma is potentially accessible to surgical excision and long-term cure.