A 53-year-old man was hospitalized because of coma appearing shortly after exposure to methanethiol. Acute, severe hemolytic anemia and methemoglobinemia developed: both were brief in duration. The likely mechanism of the hemolysis was an oxidant effect of methanethiol in a person deficient in erythrocytic glucose-6-phosphate dehydrogenase (G-6-PD). Deep coma persisted until death 28 days after exposure to the chemical agent.