The diagnostic value of the presence of anti-neuronal antibodies in serum was examined in 21 patients suspected of paraneoplastic disorders of the nervous system (NS) (group 1) and was compared to three control groups; group 2: 25 patients with a neurological disease, without cancer and no sign of paraneoplastic disorder; group 3: 27 patients with neurological disease and cancer and no signs of a paraneoplastic disorder; group 4: 94 patients with cancer and without neurological disease. In group 1, anti-neuronal nuclear antibodies were detected in eight patients (38%), in titres from 1:1000 to 1:32,000. A small cell lung cancer was present in six patients, ovarian cancer in one patient and in one patient no tumour could be detected. The neurological symptoms preceded a diagnosis of cancer in five out of eight patients. Anti-neuronal antibodies were found in the serum of two out of 94 patients (2%) from control group 3 but not in serum from any of the other control groups. These data indicate a moderate sensitivity of 38%, but a high specificity of 98.6% (95% confidence interval 95.5-99.8%) for the presence of anti-neuronal nuclear antibodies if a paraneoplastic NS disorder is suspected.