We studied 9 patients with motor neuron disease and lymphoma. The following several observations have not been recognized in the past: (1) Motor neuron syndromes are associated with either Hodgkin's disease or non-Hodgkin's lymphoma. (2) The syndromes are not restricted to lower motor neuron disorders; 8 of 9 patients had definite or probable upper motor neuron signs as well, qualifying for the diagnosis of amyotrophic lateral sclerosis. Corticospinal tracts were affected in both postmortem examinations. (3) The combination of motor neuron disease and lymphoma is often accompanied by paraproteinemia (3 of 7 patients studied), increased cerebrospinal fluid protein content (6 of 9 patients), and cerebrospinal fluid oligoclonal bands (3 of 9 patients). (4) In 2 patients, asymptomatic non-Hodgkin's lymphoma was found only because the discovery of paraproteinemia gave impetus to examine the bone marrow. (5) Patients with both upper and lower motor neuron signs (amyotrophic lateral sclerosis) may show physiological evidence of conduction block in peripheral nerves or autopsy abnormalities in peripheral nerves. The cause of this syndrome is not known. Both lymphoma and motor neuron disease could have a common cause, possibly a retroviral infection. The frequency of paraproteinemia suggests that an immunological disorder may play a role in the pathogenesis of the neurological disorder.