In addition to interleukin (IL)-6, IL-10 is considered as one of the most important cytokines regulating the proliferation and cellular characteristics of myeloma cells. It is still unclear from the clinical data how serum IL-10 levels of various stages of myeloma, are related to clinical manifestations of this disease. Several studies have reported that IL-10 affects myeloma cells by stimulating secondary signals for cell proliferation through oncostatin M (OSM) and IL-11. In experiments using human myeloma cell lines established at our laboratory, IL-10 seemed to be expressed in half of myelomas simultaneously with OSM, and to be correlated with c-maf, a transcription factor, which has been known to be overexpressed in myelomas with t(14;16)(q32;q23). In addition, IL-10 abolishes all trans retinoic acid (ATRA)-induced growth inhibition of myeloma cells. The expression and production of IL-10 in myeloma patients may be important for sub-categorization and the establishment of a case-oriented therapy.