We studied the incidence and clinical correlates of serum antibodies to GM1 and GD1a gangliosides in patients with classical amyotrophic lateral sclerosis (ALS) and other "motor nerve" syndromes. Serum antibodies to GM1 and GD1a gangliosides were measured using enzyme-linked immunosorbent assays. Our results showed that polyclonal immunoglobulin M (IgM) antibodies to the GM1 or GD1a ganglioside or both were present at serum dilutions of 1:25 to 1:4,000 in 78% (57/73) of patients with ALS. Only 8% of normal controls had similar antibodies. The pattern of serum antibody reactivity correlated with the pattern of clinical involvement in our patients. Selective reactivity to GD1a ganglioside was common when upper motor neuron signs were prominent. IgM reactivity to GM1 ganglioside was common in ALS patients with prominent lower motor neuron signs. Most patients with motor neuropathies had serum reactivity to both GM1 and GD1a gangliosides. These results provide further evidence of ongoing autoimmune processes in ALS patients. There is a strong relationship between serum antiganglioside antibodies and patterns of clinical involvement in ALS.