Elevated titers of antibodies directed at ganglioside epitopes have been associated with multifocal motor neuropathy (MMN), motor variant of chronic inflammatory demyelinating polyradiculoneuropathy (CIDP), amyolrophic lateral sclerosis (ALS), and other motor neuropathies. Anti-GM1 antibodies were measured in 675 patients: 180 age- and sex-stratified healthy blood bank controls, 132 normal controls who had full neurologic assessment including electromyography, 121 patients with definite ALS, 19 patients with pure sensory neuropathy, and 173 consecutive patient serum samples submitted for GM1 antibody testing. Antibodies to three ganglioside epitopes were determined by ELISA: IgM and IgG anti-monosialo GM1, asialo GM1, and disialo GD1b. Antibody titers for normal subjects and patients with ALS were used to determine normal values and borderline levels below which 99% of normal and 99% of ALS patient titers were found. Clinical evaluation of the next 173 consecutive patients referred for anti-GM1 antibody testing revealed 36 patients with motor neuropathies. Sera from 18 of these patients had titers above the 99% normal threshold and 14 had titers above the ALS and normal borderline threshold. All 14 with elevated sera titers were from patients with motor neuropathy or neuronopathy. Sixteen patients met the clinical and electrophysiologic criteria for MMN; 10 had elevated titers. Ten patients had the motor variant of CIDP without conduction block and three had elevated titers. Anti-IgM asialo GM1 antibodies had the highest sensitivity and specificity. High-titer IgM antibodies against monosialo GM1 occurred only in patients with various forms of pure motor neuropathy (100% specificity). The sensitivity was 50% for this referral-based population.