Several reports have documented infiltration of many monocytes in renal cell carcinoma (RCC). Since few studies have examined the relationship between monocyte infiltration and clinical prognosis in RCC, we clinically investigated this relationship by semi-quantitative analysis of monocyte infiltration and tumor angiogenesis. The following six parameters were measured immunohistologically in 98 RCC patients who underwent nephrectomy between 1987 and 1997: tumor-associated macrophage (TAM), microvessel density (MVD), S-100 protein-positive cells (S-100(+) cells), HLA-DR-positive cells, apoptosis index and proliferative index (PI). We then assessed intercorrelations among parameters and correlations to prognosis. Significant positive correlations were identified for TAM, MVD and PI, with a tendency for higher parameter values to reflect poorer prognosis. The prognosis of patients without metastasis was poor for the high TAM group even when levels of MVD were low. These findings suggest that TAM facilitates the growth of RCC via angiogenesis and other mechanisms. Prognosis was significantly better in metastatic RCC patients who underwent interferon-alpha (IFN-alpha) therapy when the levels of S-100(+) cells were high. Nonetheless, the levels of S-100(+) cells among these IFN-treated patients did not correlate with other parameters, and none of the other parameters correlated with prognosis. One of the antitumor effects of IFN-alpha for RCC could therefore be mediated by dendritic cells.