The recent discovery that defects in the gene encoding copper-zinc superoxide dismutase (SOD1) are associated with some cases of familial motor neuron disease has heightened interest in the possibility that free radical mechanisms may contribute to selective motor neuron injury. Sporadic and familial motor neuron diseases are clinically and pathologically very similar and may share common pathophysiological mechanisms. Thus the role of free radical mechanisms as a contributory factor to motor neuron injury in the common sporadic form of motor neuron disease requires urgent exploration, particularly as this may provide an avenue for therapy aimed at retarding pathological progression. We investigated oxidative damage to proteins in the lumbar spinal cord by quantifying the protein carbonyl level from 19 patients with sporadic motor neuron disease, 8 neurologically normal control subjects, and 11 neurological disease control subjects, most of whom had slowly progressive neurodegenerative disease. In sporadic motor neuron disease the mean protein carbonyl level in the spinal cord was increased by 119% (p < 0.02) compared to normal control subjects and by 88% (p < 0.04) compared to the neurological disease control subjects. These data contribute to the emerging evidence that oxidative damage may play a contributory role in the neuronal death in sporadic motor neuron disease. This mechanism may be particularly important in a subset of patients with motor neuron disease.