The paraneoplastic syndrome of erythrocytosis is associated with a variety of neoplasms including renal adenocarcinoma, cerebellar hemangioma, and hepatoma. We now report the characterization of the biological and molecular features of an erythropoietin-secreting human renal adenocarcinoma, designated RCCEp+. Serial transplantation of the tumor in athymic mice resulted in a dramatic increase in hematocrit and serum erythropoietin concentration. Growth in vitro was accompanied by a constant rate of erythropoietin secretion. Karyotype analysis demonstrated several unusual features, including the absence of 3p deletions and near tetraploidy. Erythropoietin mRNA was demonstrated by Northern blot both in freshly excised tumor and in tumor cells growing in vitro. Erythropoietin secretion was constitutive and was not induced either by cobalt or hypoxia. Southern blot analysis revealed no rearrangement of the erythropoietin gene in the tumor. Interestingly, in situ hybridization demonstrated erythropoietin mRNA in only a small population of the tumor cells. Further studies of RCCEp+ should prove useful in elucidating the molecular basis for this paraneoplastic syndrome.