Type 1 antineuronal nuclear autoantibody (ANNA-1, also known as "anti-Hu") is a marker of neurologic autoimmunity that is highly associated with small-cell lung carcinoma (SCLC). To determine the spectrum of symptoms and signs as well as the frequency of cancer in adult patients who are seropositive for ANNA-1, we reviewed 162 sequential patients (67% female) identified as ANNA-1-positive in a comprehensive immunofluorescence screening test. In 21% of these patients, the antibody test requested by the physician was not ANNA-1. By the end of the follow-up period, cancer had been found in 142 patients (88%). Ten of these lacked evidence of SCLC (4 had prostate carcinoma, 3 breast carcinoma, 1 both prostate carcinoma and melanoma, 1 lymphoma, and 1 squamous-cell lung carcinoma). Of the 132 patients(81%) with proven SCLC, 17 had one or more coexisting malignant neoplasms (6 had renal carcinoma, 4 another lung primary carcinoma, 3 prostate carcinoma, 3 breast carcinoma, and 4 assorted neoplasms). The diagnosis of SCLC in 128 patients(97%) followed the onset of paraneoplastic symptoms. SCLC was identified in 10 patients by chest MRI after an equivocal chest radiograph or CT; in 28 by bronchoscopy, mediastinoscopy, or thoracotomy; and in 7 at autopsy. Neurologic signs in decreasing frequency were neuropathy (sensory> mixed somatic > autonomic > cranial [especially cranial nerve VIII] > motor), cerebellar ataxia, limbic encephalitis, polyradiculopathy, associated Lambert-Eaton myasthenic syndrome, myopathy, myelopathy, opsoclonus/myoclonus, motor neuronopathy, brachial plexopathy, and aphasia. Nineteen patients had a solely gastrointestinal initial presentation, including gastroparesis, pseudo-obstruction, esophageal achalasia, or other dysmotility. We conclude that seropositivity for ANNA-1 can expedite the diagnosis and treatment of otherwise occult cancer in patients, especially tobacco abusers, with varied neurologic and gastroenterologic presentations. The search for SCLC should not end on discovering a different neoplasm.