The Lewy body (LB) is a cytoplasmic neuronal inclusion that is constantly found in the substantia nigra and locus ceruleus, and other specific brain regions in Parkinson's disease (PD). In the present study, the significance of non-parkinsonian cases in which the LBs were an incidental finding was investigated. In 1,452 autopsy series (age range, 1-89 years), 27 incidental cases of LBs and 21 patients with PD were found. The age-specific prevalence of incidental LBs steadily increased from 0.7% to 6.5% between the sixth and ninth decades. The distribution of LBs in incidental cases was similar to that in PD. The number of neurons in the substantia nigra and locus ceruleus was determined in 8 incidental cases (age range, 60-81 years), 9 patients with PD (range, 56-81 years) and 9 control subjects (range, 60-81 years). In incidental cases, the average numbers of pigmented neurons in the substantia nigra and locus ceruleus was 83% and 77% of those in control subjects, respectively. In PD, the average numbers of pigmented neurons in the substantia nigra and locus ceruleus was 28% and 19% those of controls, respectively. However, the number of nonpigmented neurons in the substantia nigra was not reduced in incidental cases and PD. From the above findings, it is suggested that there exists a close relationship between the occurrence of LBs and aging. However, in the brains of elderly patients, incidental LBs were considered to be pathological findings rather than simple aging phenomenon. Therefore, it has been suggested that incidental cases may represent early and presymptomatic cases of PD.