A 74-year-old man had a paraneoplastic motor neuron disease mimicking amyotrophic lateral sclerosis. He had an elevated erythrocyte sedimentation rate, other laboratory abnormalities, and a previously undiagnosed renal cell carcinoma. Four months after nephrectomy, his strength had improved and he had no fasciculations. Seven other patients with cancer and motor neuron disease improved or stabilized after tumor treatment. Even though it is rare, paraneoplastic motor neuron disease is important to diagnose because it may be treatable.