Metastatic renal cell carcinoma (MRCC) is one of the malignancies that are sensitive to immunotherapy. However, the underlying immune inhibitory factors such as myeloid-derived suppressor cells (MDSCs) might restrain the efficacy of immunotherapy. The present study investigates the clinical efficacy of cytokine-induced killer (CIK) cell therapy in patients with MRCC and explores whether the levels of peripheral MDSCs are associated with the prognosis of patients receiving this therapy. Twenty-nine patients with measurable MRCC were treated with an adoptive transfer of autologous CIK cells, followed by 5 consecutive days of interleukin-2 administration. The tumor response and 1-year survival were observed. The proportion of MDSCs in the peripheral blood was detected, and the correlation of MDSCs with prognosis was analyzed. Of 29 evaluable patients, no complete responses were seen; 4 patients exhibited a partial response (13.8%), 18 patients displayed stable disease (62.1%), and 7 patients showed progressive disease (24.1%). Twenty patients (69.0%) were alive 14.8-41.4 months at the time of the last follow-up (median follow-up=20.2 mo). The 1-year survival was 82.8% (24/29). Peripheral blood MDSCs were elevated in almost all MRCC patients and decreased after CIK-cell infusion. Subgroup analysis indicated that patients with a relatively low proportion of MDSCs exhibited prolonged survival. In conclusion, our data suggest that transfusion of autologous CIK cells can induce regression of MRCC, and MDSCs can serve as a potential marker for the prognosis of patients receiving a CIK-based therapy.