The prevalence of neurological abnormality was determined among Cree Indians of northern Quebec who were exposed to methylmercury through contamination of local fish. A medical team examined each subject, without knowledge of the subjects' methylmercury exposure, and assessed nystagmus, co-ordination and gait, tremor, movements and reflexes, sensation, stereognosis, 2 point discrimination, visual fields and hearing. Tremor, incoordination and abnormal reflexes were the most prevalent neurological abnormalities, but other major manifestations of methylmercury toxicity (impairment of peripheral sensation, astereognosis and concentric constriction of visual fields) were seen infrequently. Abnormalities in Quebec were much less severe than in previous reported outbreaks of methylmercury poisoning. Since no single clinical syndrome is pathognomonic of methylmercury poisoning, the presence of neurological abnormality does not establish that it is causally related to methylmercury exposure.