An alteration within the mitochondrial DNA (mtDNA) has been hypothesized to underlie the deficiencies in mitochondrial complex I activity observed in the platelets, striatal muscle, and brain tissue of individuals with Parkinson's disease. Here we utilized the polymerase chain reaction (PCR) to analyze mtDNA obtained from the platelets of nonmedicated patients with early Parkinson's disease (n = 8) and aged-matched controls (n = 6) for the presence of deletion(s) or addition(s) equal to or greater than 50-100 base pairs. Initial attention was focused upon detecting a 4.977 kb deletion previously found in the brains of parkinsonian patients and some aged controls. Indeed, a large deletion of approximately 5.0 kb was observed in the platelet mtDNA from all parkinsonian individuals. However, this defect was also found in all age-matched controls as well as in a group of young healthy subjects (n = 5). In addition, we searched for the presence of smaller changes in platelet mtDNA from parkinsonian patients by PCR analysis of four mtDNA segments that code for seven of the complex I polypeptides. No large deletions or additions were detected within these four regions of mtDNA in any of the disease or age-matched control samples. We conclude that (a) a 4.977 kb deletion is apparently present in a subpopulation of platelet mtDNA from all individuals, and (b) no macrosequence alteration in mtDNA is likely to underlie the deficiency in complex I activity reported in platelet mitochondria from parkinsonian patients.