Several recent reports indicate that occupational exposure to electric and magnetic fields may be associated with increased risk of neurodegenerative diseases. To address that hypothesis, we analyzed data from a cohort study of electric utility workers. We examined exposure to magnetic fields, assessed as duration of work in exposed jobs and through an index of cumulative exposure based on magnetic field measurements, in relation to mortality from Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis, considering both underlying and all mentioned causes of death. Adjusted mortality rate ratios based on Poisson regression models indicate no association between magnetic fields and Parkinson's disease and little support for an association with Alzheimer's disease mortality. Mortality from amyotrophic lateral sclerosis was positively associated with duration of work in exposed jobs [rate ratio = 2.0, 95% confidence interval (CI) = 0.7–6.0; and rate ratio = 3.1, 95% CI = 1.0–9.8, based on underlying cause for 5− < 20 years and ≥20 years vs < 5 years, respectively], as well as with cumulative magnetic field exposure with a ≥20-year lag (rate ratio = 2.3, 95% CI = 0.8–6.6; and rate ratio = 3.0, 95% CI = 1.0–9.2, for exposure in the middle and upper intervals relative to the lowest interval, respectively). (Epidemiology 1998;9:398–404)