We attempted to survey with serum protein electrophoresis all patients with peripheral neuropathy of unknown cause presenting at our institution within 1 year to determine the prevalence of monoclonal serum proteins in this group. Eighty-four percent of these patients were evaluated, and 10% of these had a monoclonal serum protein. This figure was statistically different from that obtained in two community studies in the literature. Because these patients often present difficult diagnostic problems, serum protein electrophoresis is necessary in the evaluation of cryptogenic neuropathies.