Primary lateral sclerosis is an uncommon, distinct clinical entity. We report a patient with primary lateral sclerosis in whom investigations revealed an IgM monoclonal gammopathy, raised CSF protein and persistently high ESR. A number of reports suggest that lymphoproliferative disorders, paraproteinemia and clinico-pathological syndromes mimicking motor neuron diseases may be associated. We discuss the clinical features noted in our patient in relation to these reports, and the possible pathogenetic mechanisms.