The pathogenesis of neuronal degeneration in both sporadic and familial amyotrophic lateral sclerosis (ALS) associated with mutations in superoxide dismutase may involve oxidative stress. A leading candidate as a mediator of oxidative stress is peroxynitrite, which is formed by the reaction of superoxide with nitric oxide. 3‐Nitrotyrosine is a relatively specific marker for oxidative damage mediated by peroxynitrite. In the present study, biochemical measurements showed increased concentrations of 3‐nitrotyrosine and 3‐nitro‐4‐hydroxyphenylacetic acid in the lumbar and thoracic spinal cord of ALS patients. Increased 3‐nitrotyrosine immunoreactivity was observed in motor neurons of both sporadic and familial ALS patients. Neurologic control patients with cerebral ischemia also showed increased 3‐nitrotyrosine immunoreactivity. These findings suggest that peroxynitrite‐mediated oxidative damage may play a role in the pathogenesis of both sporadic and familial ALS.