To investigate the relationship between the serum soluble interleukin-2 receptor (sIL-2R) and the stage and prognosis of breast cancer. Using the method of sandwich ELISA, the preoperative and postoperative sIL-2R in 37 cases of breast cancer was measured, 13 patients with benign disease of breast and 40 normals were also measured. The serum sIL-2R levels in patients with breast cancer were significantly higher than that of the benign disease of breast and the normal controls. The serum sIL-2R about twenty days after operation was significantly lower than that before operation. The sIL-2R in patients with breast cancer was correlative to the clinical stage. The serum sIL-2R in stage Ⅲ patients was remarkly higher than both stage Ⅰ and Ⅱ, its sIL-2R about twenty days after operation remained in a high level. This results showed that expression of the serum sIL2R is correlative to the clinical stage and prognosis of the patients with breast cancer, it can be used as a marker of differentiating the benign from malignant disease of breast.