['Hemangioblastomas express erythropoietin and the patients often present with polycythemia.', 'Serum erythropoietin was measured using a commercial immunoassay, a functional erythropoietin assay and iso-electric focusing.', 'Despite the polycythemia, serum erythropoietin remained low, while a functional erythropoietin-assay showed a 4-5 higher activity in serum compared to the immunoassay. Iso-electric focusing of serum erythropoietin indicated overrepresentation of highly sialylated erythropoietin isoforms produced by the tumor. As a result, altered affinity of the monoclonal antibody used in the immunoassay for the hypersialylated isoforms was suggested.'] Is an abnormally glycosylated isoform of erythropoietin in hemangioblastoma associated with polycythemia?