To clarify the relationship between variation in mtDNA and the development of cardiomyopathy (CM), the complete sequences of mtDNAs of two brothers with dilated CM were compared with those of 181 patients who had CM and with those of 168 control subjects. Five patients with CM shared a novel homoplasmic point mutation (G12192A tRNA(His)), and all of them demonstrated the evolutionarily related D-loop sequence. The results suggest that this novel mutation originated from the same ancestor and that its presence strongly predisposes carriers to CM.