In some patients, motor neuron disease (MND) is associated with a paraprotein, raising the question that there may be a relationship between MND and monoclonal gammopathy. We analysed the presence of monoclonal immunoglobulin in the serum from 53 patients with MND using immunofixation in cellulose acetate membrane. The incidence of paraproteins in 6 cases (11.3%) of patients with MND exceeds values found with cellulose acetate membrane supports in reported populations (below 1%), even when age was also considered. Monoclonal components were IgG (33%), IgM (33%) and IgA (33%). In six cases, four showed typical changes of amyotrophic lateral sclerosis and other two patients were spinal progressive muscular atrophy (SPMA) in autopsy. No malignancy was detected in all cases. These results corroborate the concept of a probable association between MND and benign monoclonal gammopathy (plasma cell dyscrasia). A patient with SPMA had transient elevation of IgG-GM1 antibodies. Other five patients with M-proteinemia did not show elevated serum anti GM1 antibodies.