Southern hybridisation demonstrates deleted mitochondrial DNAs (mtDNAs) in muscle but not in blood in a subgroup of patients with mitochondrial myopathy. The polymerase chain reaction (PCR) was used to search for low levels of rearranged mitochondrial DNAs in blood in 24 patients with mitochondrial myopathy, and 15 asymptomatic relatives, all of whom have no detectable abnormality on restriction enzyme analysis of blood mitochondrial DNA. In eight patients and two of their relatives, PCR products were obtained consistent with deletions of mitochondrial DNA. The presence or absence of a deletion was correctly predicted in 10 out of 11 patients from whom information was available from muscle DNA. No false positives were obtained in 43 controls. PCR analysis of blood may be applicable as a non‐invasive screening test of affected individuals and in carrier detectio