In a survey to determine the occurrence of Levy bodies in the elderly, the prevalence rate of Lewy body formation was found to be critically dependent on the psychiatric status of control cases. In 131 controls between 51 and 100 years screened to exclude psychiatric and neurological disorders, the Lewy body prevalence rate was 2.3%, but inclusion of cases with psychiatric disorders other than Alzheimer's disease increased the prevalence rate to 9%. An age-related decline in substantia nigra and locus coeruleus neuron numbers was observed in the control group. Brain stem Lewy body formation (found in 3 cases) was not necessarily linked with neuron loss in substantia nigra, though in two of the cases significant locus coeruleus neuron loss was observed. Within the control group, there was no obvious relationship of Lewy body formation to the extent of Alzheimer-type pathology. These findings are compatible with the disease specificity of Lewy bodies and suggest that Lewy body disorders have a relatively short preclinical phase in which Lewy body formation may precede both locus coeruleus and substantia nigra neuron loss. The increase of Lewy body positive cases found when individuals with psychiatric disorders are included in the population surveyed supports the emerging concept of a spectrum of Lewy body diseases ranging from purely psychiatric disorders through combined psychoneurological or neuropsychiatric symptoms, to the classically described neurological disorders of Parkinson's disease.