Possible occupational risk factors for neurodegenerative diseases were investigated. Mortality from presenile dementia (PSD), Alzheimer's disease (AD), Parkinson's disease (PD), and motor neuron disease (MND) was examined for 27 states in the National Occupational Mortality Surveillance (NOMS) system for the period 1982 through 1991. NOMS was a death certificate based data collection system on which the underlying and contributing causes of death, usual occupation, and various demographic variables were coded. Proportionate mortality ratios (PMRs) were computed by comparing the proportion of deaths for all decedents who were at least 15 years (yr) old, with any mention of PSD, AD, PD, or MND on the death certificate, to all causes of death. A total of 130,420 deaths from the four neurodegenerative diseases were recorded during the study period. Occupational clustering for specific diseases was found in specific occupations such as agricultural workers (PSD, PD, and MND), electrical workers (PSD, AD, PD, and MND), and construction workers (PSD and MND). There was also a cluster for religious workers and counselors with increased mortality from PSD, AD, and PD. Another common pattern was for lawyers, judges, and archivists who showed increased mortality from AD, PD, and MND. A large number of jobs in which workers had probable exposure to solvents showed increased mortality from the various diseases. Occupations with statistically significant increased proportionate mortality for deaths occurring between the ages of 15 and 55yr were also examined since a relatively early age at death might indicate that an occupational factor was involved in the etiology or pathogenesis of the diseases. These groups generally had only small numbers of cases. More occupations with significantly increased PMRs were found for MND than the other diseases. Included in this group were firefighter, teachers, machinists, military personnel, veterinarians, excavation machine operators, janitors, and packaging machine operators, PMRs = 318, 214, 318, 188, 1108, 538, 235, and 985, respectively. The authors conclude that this study indicates that neurodegenerative diseases occur more frequently in some occupations than in others. This finding which may indicate occupational risk factors should be investigated further.