The results of a study on the content of erythropoietic‐stimulating factor in the serum, urine and extracts from tumour tissue and normal renal tissue from a patient with erythrocytosis and hypernephroma of the left kidney are reported. Extracts from the solid parts of the tumour showed a high concentration of this factor. It seems reasonable to assume that this factor, which is presumed to be responsible for the patient's erythrocytosis, was produced by the tumour cells.Increased concentrations of erythropoietic‐stimulating factor were also revealed in the serum and urine after operation. Analyses of the metastases were not performed, but itseems reasonable to assume that these also produced an erythropoietic‐stimulating f