The presence of circulating antineuronal antibodies has been associated with paraneoplastic neurological syndromes (PNS). Ri antibodies are often associated with lung or breast cancer, but the prevalence of such antibodies in large cancer materials is largely unknown. We used a highly sensitive immunoprecipitation assay to study the level of Ri antibodies in blood samples from 200 patients with small cell lung cancer (SCLC), 253 patients with breast cancer and 557 patients with ovarian cancer. Two hundred blood donors and six Ri positive PNS patients served as controls. The recombinant antigen used in the immunoprecipitation assay was radiolabeled by a coupled in vitro transcription and translation (ITT) technique, enabling low levels of antibodies to be detected. None of the blood donors contained Ri antibodies, whereas all of the sera from the PNS patients were positive. Ri antibodies were present in 4.5% of the patients with SCLC, 0.8% of the patients with breast cancer and in 0.2% of the patients with ovarian cancer. Retesting of the Ri positive samples with immunofluorescense and immune blot showed that the immunoprecipitation technique was more sensitive than the other immune assays. Ri antibodies were not associated with PNS in the patients with breast or ovarian cancer. Neurological data were not available for the SCLC patients, but in these, Ri antibodies were not associated with survival.