We have explored MICA/B expression and its relationship with innate inflammatory infiltrate in renal cell carcinoma (RCC). The expression of MICA/B, CD16, CD56, and CD68 in 140 RCC lesions contained in a tissue microarray (TMA) was investigated by immunohistochemistry. MICA/B gene and protein expressions in Caki-1 cells were analyzed by reverse transcription-polymerase chain reaction and flow cytometry, respectively. Natural killer (NK) cells were studied by flow cytometry. All the RCC lesions (n = 140) were MICA/B-positive. MICA/B was mainly expressed in the cytoplasm of tumor cells, whereas stromal cells were negative. Renal cell carcinoma lesions showed low NK cell infiltration, although they were rich in CD16(+)CD56(-) cells, strongly resembling macrophages. CD16(+) macrophage infiltration was more frequently detectable in metastatic lesions compared with primary tumors (P = .0223) and was associated with poor RCC differentiation (P = .007). To investigate mechanisms potentially underlying the lack of NK cells infiltration into MICA/B-positive RCC lesions, we used Caki-1 RCC cells. Caki-1 expressed MICA and MICB genes. However, MICA protein was not detectable in Caki-1 cells, whereas MICB protein was detectable in their cytoplasm and on the cell membrane. Coculture of peripheral blood mononuclear cells with Caki-1, K562, HCT116, respectively, resulted in CD56(+)CD16(+) NK cells deletion without affecting CD56(+)/CD16(-) NK subset and immature NK cells generated in vitro from CD34(+) cells. Natural killer cell apoptosis seemed to be preferentially triggered by cancer cells because HLA-A0201(+) NK cells were only marginally affected by allogeneic HLA-A0201(-) peripheral blood mononuclear cells. Caki-1 cell-mediated NK cell apoptosis was reduced by an anti-beta(2)-integrin (CD18) monoclonal antibody but was NKG2D-, granule exocytosis-, and caspase-independent.