A retrospective study of 269 patients with a clinical diagnosis of Parkinson's syndrome was carried out. Seventy-eight were selected as cases of Parkinson's disease on the basis of generally accepted criteria. Of these, 73 were found to have Lewy bodies and cell loss in the substantia nigra, two had the pathological changes of striatonigral degeneration, two had extensive neurofibrillary tangles and nigral cell loss compatible with post-encephalitic parkinsonian syndrome, and one had Lewy bodies and striatonigral degeneration. Pathological examination of the remaining brains revealed Lewy bodies with cell loss in the nigra, a different pathological diagnosis, or normal basal ganglia. No unclassifiable 'senile' patients, or cases with neurofibrillary tangle degeneration or extranigral pathology of unknown cause were found. As a result of these findings clinico-pathological diagnostic criteria for Parkinson's disease are proposed. All patients with this diagnosis had Lewy bodies in surviving neurons of two unilateral 7 microns sections of the mid-part of the substantia nigra. The frequency of Lewy bodies in multiple system atrophy, Steele-Richardson-Olszewski syndrome and Alzheimer's disease was low and similar to that in age-matched 'normal' controls. Pathological findings were compatible with the notion that these individuals have preclinical or coincidental Parkinson's disease.