Mitochondrial genetic modifying factors have been suspected in several autosomally inherited diseases. The congenital variant of myotonic dystrophy, in which there is a striking maternal inheritance pattern, is a likely candidate disease. To investigate this possibility, we sequenced completely the mitochondrial genome in 2 patients with congential myotonic dystrophy. Comparison of the two sequences with control data failed to reveal a specific nucleotide variant or length variant in this disease. We conclude that a mitochondrial genetic modifying factor is not present in congenital myotonic dystrophy.