Our purpose was to elucidate the clinical roles of the "immuno-proteosome," which is involved in the accelerated pathway of the major histocompatibility complex (MHC) class I-restricted antigen presentation system, in renal cell carcinoma (RCC). The relative expression of six proteosome subunits (existing subunits X, Y, and Z and immunoproteosome subunits LMP7, LMP2, and MECL1) in 54 RCCs was investigated using RT-PCR analysis and was compared with clinicopathological measures, including patient outcome. Expression of the LMP7 and LMP2 genes was significantly low in high-grade tumors, and that of the LMP7 and MECL1 genes was significantly low in high-stage tumors. Low levels of LMP7, LMP2, and MECL1 expression were strongly associated with shortened survival (LMP7: P = 0.0002, LMP2: P < 0.0001, MECL1: P < 0.0047). The levels of subunits X, Y, and Z had no significant correlation with those measures. These findings suggest that RCCs with low level of immuno-proteosome subunit expression have a disorder in their antigen-presentation system. As a consequence, they may escape from immune surveillance and worsen patient outcome.