We studied 26 patients with both motor neuron disease and lymphoproliferative disease (LPD). Twenty-three patients had definite or probable upper motor neuron signs; none had electrophysiologic evidence of motor neuropathy. LPD syndromes comprised Waldenström's macroglobulinemia, multiple myeloma, chronic lymphocytic leukemia, follicular cell lymphoma, and Hodgkin's disease. In all but one patient, the cause of disability or death was neurologic. LPD was confined to bone marrow in 14 patients; eight of 14 had monoclonal paraproteinemia. One patient had LPD discovered at autopsy. Treatment of LPD in 20 patients resulted in neurologic improvement in 1 patient and arrest in another; both had progressive spinal muscular atrophy. Eleven patients were worse and 13 died. At least 30 cases have been reported from other centers, bringing the total to 56. Among the unusual reported concomitants were POEMS (polyneuropathy, organomegaly, endocrinopathy, myeloma, and skin changes) syndrome of myeloma and angiotropic lymphoma.