The purpose of this study was to define the cause of severe gastrointestinal motor dysfunction in 7 patients with lung cancer. Six patients had small cell carcinoma and 1 patient had pulmonary carcinoid. Their ages ranged from 58 to 74 yr. All had intestinal pseudoobstruction and obstipation/constipation; 6 of 7 patients had gastroparesis; 4 of 4 patients had esophageal peristaltic abnormalities; and 2 patients had neurogenic bladders, autonomic insufficiency, and peripheral neuropathy. Five of 7 patients had dilated small bowel with 4 of them showing slow transit of barium; 2 of 7 patients had dilated colons; and 3 of 7 patients had slow colonic transit. Five patients died 4-9 mo after onset of gastrointestinal symptoms, and 2 survived. Post-mortem or surgical samples of the esophagus, stomach, small bowel, and colon showed neuron and axon degeneration and dropout, lymphoplasmacytic infiltration, and glial cell proliferation within the myenteric plexus of 6 patients. The antrum from the seventh patient had inflammatory cells within the myenteric plexus but without neuron dropout. Neuron numbers were significantly less than normal in each area of the gastrointestinal tract. Thus, we conclude that lung cancer may be complicated by severe gastrointestinal motor dysfunction resulting from visceral neuropathy of the myenteric plexus, a paraneoplastic effect of the cancer.