Parkinson's disease is a nosological entity of unknown origin for which, in some cases, a possible pathogenetic role for mitochondrial dysfunction has been postulated. Two young onset parkinsonian patients with mitochondrial DNA (mtDNA) deletions in skeletal muscle are reported on. Patient 1 also presented with increased blood creatine kinase and lactate concentrations and a family history which included a wide range of phenotypes affecting multiple systems. Patient 2 presented with multiple symmetric lipomatosis. Histopathological investigation showed ragged red fibres and COX negative fibres in muscle biopsies from both patients. The data support the hypothesis that mitochondrial DNA mutations may occur in some cases of parkinsonism, suggesting that a diagnosis of a mitochondrial disorder should be considered in the presence of consistent family history and clinical symptoms.