We examined the family history and associated diseases in 58 patients with amyotrophic lateral sclerosis (ALS), as well as the T-cell phenotypes and functions in 46 consecutive patients with this disorder. A family history of thyroid disease was present in 19%, and an additional 21% of patients described family members with other possible autoimmune disorders. In 19% of the patients with ALS either past or present thyroid disease was documented. Eleven of 47 additional patients with ALS had significant elevations of microsomal and/or thyroglobulin antibody levels. The T-cell phenotypes and functions were comparable in the ALS and control groups, with the exception of the presence of Ia antigen. In patients with ALS, 11.9% of the T cells were positive for the la antigen, while in both a normal control population and a non-ALS neurologic disease population, only 6.4% of T cells have this antigenic determinant. These data support involvement of autoimmune mechanisms in ALS.