No etiological diagnosis was obtained for 35 of 209 patients studied. Mean patient age was 37.7 years and the preferentially affected age range was 20 to 50 years. Laboratory investigation was more extensive among these patients than among patients with a defined diagnosis. An 11:1 ratio was obtained between normal and abnormal laboratory tests. The major topographic diagnoses were: axonal polyneuropathies, 5 cases; multineuropathies and radiculopathies, 3 cases each. The present findings are discussed and compared with the literature concerning neuropathies of undefined diagnosis.