A mitochondrial myopathy associated with multiple deletions of mitochondrial DNA has been identified in pedigrees showing an autosomal dominant mode of inheritance. We report the first two British kindreds with this disorder, and two sporadic cases. The families exhibited some unusual clinical features, including pigmentary retinopathy and tremor; the latter was levodopa-responsive and associated with rigidity and micrographia in one family. Members of one pedigree and both sporadic patients had a peripheral neuropathy and nerve biopsy showed marked axonal degeneration. Post-mortem examination of one patient without parkinsonism showed severe neuronal loss in the substantia nigra.