Among Guamanian natives, serum IgA and IgG levels were found to be higher than normal in amyotrophic lateral sclerosis (ALS); serum IgA was higher and IgM lower than normal in parkinsonism-dementia (PD). IgA levels increased with age in ALS, PD, and normal subjects; IgG increased with age in ALS and IgM decreased with age in PD. Serum immunoglobulin (Ig) levels did not correlate with the duration of either disease. Immunodeficient ALS and PD patients had higher IgM and lower IgA levels than the other ALS and PD patients. Neither differences in viral antibody titers nor the presence of autoantibodies or circulating immune complexes could account for the variations in serum Ig levels between patients and controls. We conclude that differences in serum Ig levels in ALS and PD patients are probably due to repeated infections and abnormal immunoregulation accompanying immunodeficiency during the course of ALS and PD, rather than to a specific antiviral or autoimmune response.