We report the detection of erythropoietin mRNA in the tumor from a patient with neuroblastoma and erythrocytosis. A 2-year-old girl with neuroblastoma presented with hemoglobin 21.3 g/dL, red blood cells 8.03 x 1012/L, and hematocrit 0.641. The serum erythropoietin level was 26.54 mU/mL. The hemoglobin, hematocrit, and serum erythropoietin level were within normal ranges 3 months after surgical excision of the tumor. RT-PCR analysis showed that erythropoietin mRNA expression in the tumor tissue was as high as that of normal renal tissue. These results conclusively demonstrated that the tumor was the site of the ectopic erythropoietin production.