Serum and CSF from 48 patients with amyotrophic lateral sclerosis and six patients with late-onset postpoliomyelitis progressive muscular atrophy were investigated for the presence of antibody to poliovirus types 1, 2, and 3, coxsackie viruses B3 and B4, influenza A, measles, rubella, mumps, herpes simplex types 1 and 2, cytomegalovirus, varicella-zoster, and Toxoplasma gondii. These results were compared with those from 53 control patients with neuromuscular disease matched for age, sex, race, and poliovirus vaccine exposure. There was no difference either in distribution of serum or CSF antibody titers or the geometric-mean antibody titers. There was no evidence suggesting the presence of locally produced specific viral antibody within the CNS to any of the agents studied. In particular, there was no serological evidence to suggest an association between persistent infection with any poliovirus type and amyotrophic lateral sclerosis or late-onset postpoliomyelitis progressive muscular atrophy.