Previous reports indicate that motor neuron disease (MND) may rarely be associated with systemic cancer. We have encountered 14 patients with MND and cancer who formed three distinct groups. Group 1: Three patients developed a rapidly progressive MND, less prominent symptoms of involvement of other areas of the nervous system, and anti-Hu antibodies. Group 2: Five women developed signs of upper motor neuron (UMN) disease, initially resembling primary lateral sclerosis (PLS), and breast cancer. In 4, symptoms of UMN occurred within 3 months of cancer diagnosis or tumor recurrence. They had no metastases or spinal cord compression. Serum anti-neuronal antibodies were negative. Three patients are alive (follow-up of 156, 15, and 12 months), and 2 remain without lower motor neuron signs. Group 3: Six patients developed MND resembling amyotrophic lateral sclerosis between 47 months before and 48 months after their cancer diagnosis. In group 1, the MND associated with the anti-Hu antibody is unequivocally paraneoplastic. In group 2, the proximate onset of MND with the diagnosis of cancer or its recurrence, its pure or long-lasting UMN signs, and its association with breast cancer, suggest that the disorder may be paraneoplastic. Although for most cancer patients who develop MND the occurrence of both disorders is probably coincidental, in some patients with MND a careful search for an underlying cancer is warranted (ie, patients in groups 1 and 2).