Anti-GM1 IgM antibodies were found in 23% of 56 patients with motor neuron disease (MND), in 19% of 69 patients with neuropathy and in 7% of 107 controls with other neurological and non-neurological diseases. Most patients had anti-GM1 IgM titers of 1:80 or less, while slightly higher titers (up to 1:640) were found in 1 patient with MND and 2 with neuropathy, and very high titers (1:20,480) were restricted to a patient with MND and an IgMk M-protein reacting with GM1, GD1b and asialo-GM1. Anti-GM1 IgM antibodies may be a normal constituent of the human antibody repertoire but their frequency and, at times, their levels are higher in patients with MND or neuropathy. The possible pathogenetic role of these antibodies remain to be established.