Myeloid-derived suppressor cells(MDSCs) have been reported to be induced by inflammation, to suppress immune function, and promote tumor progression, and to be found in circulating blood, tumor tissue, and lymph nodes. MDSCs were found in pleural effusions and ascites of patients with malignant diseases and these results are described in this report. Changes in the levels of MDSCs during clinical responses to cancer chemotherapy are also reported. MDSC levels in malignant effusions have also been shown to change with the levels of MDSCs in peripheral blood and with clinical responses. MDSCs seem to play an important role in inflammation that is strongly related to advancing malignant diseases. The results also suggested that MDSCs may be reduced after effective chemotherapy, which may be effective as an adjuvant treatment with cancer immunotherapy.