Severe gastrointestinal dysmotility is a newly recognized paraneoplastic syndrome that occurs with small-cell lung carcinoma. Thirty-four patients with small-cell carcinoma, of whom 5 had chronic intestinal pseudoobstruction and 29 had no digestive symptoms, were studied serologically. Four of the 5 patients with gut dysmotility had immunoglobulin G antibodies reactive with neurons of the myenteric and submucosal plexuses of jejunum and stomach in an indirect immunofluorescence assay. Antibodies of this type were not found in any of the 29 patients who had no gut dysmotility, nor were they found in patients with chronic idiopathic intestinal pseudoobstruction (n = 8), ovarian cancer (n = 20), or epilepsy (n = 4) or in normal subjects (n = 9). In 4 of the patients with paraneoplastic pseudoobstruction, antibodies in highly diluted serum (1:4000-1:8000) bound selectively to nuclei and cytoplasm of neuronal elements in the gut. This novel autoantibody activity suggests that intestinal pseudoobstruction occurring in patients with small-cell carcinoma may have an autoimmune basis. From a clinical standpoint, serological testing offers a simple means for determining which patients with gut dysmotility syndromes may have associated small-cell carcinoma, thereby enabling earlier diagnosis and treatment of the tumor.