A cornerstone of mitochondrial genetics, strict maternal inheritance, has been challenged recently by the study of a patient with mitochondrial myopathy due to a sporadic 2bp deletion. The mitochondrial DNA (mtDNA) harboring the mutation was paternal in origin, whereas the patient's blood was identical to the maternal genotype. To determine whether this is a common phenomenon, we studied mtDNA sequence variation between muscle and blood from 35 patients with sporadic mitochondrial myopathies, but detected no evidence of paternal mtDNA transmission. Our findings suggest that paternal transmission of mtDNA is rare and should not alter our genetic advice to families.