Possible prognostic utility of CD44 in renal cell carcinoma (RCC) prompted a comparison of its expression in primary and metastatic RCC. A total of 164 paraffin-embedded tissues of primary RCC and metastatic RCCs from 125 patients were immunostained with CD44 (standard form) antibody. It consisted of 86 primary RCCs (50 with subsequent metastasis [MET+] and 36 with no known metastasis [MET-]) during follow up and 78 metastatic RCCs (39 metastatic RCCs only and 39 with matched RCC primary from RCC MET + category). Immunoreactivity for CD44 was scored semiquantitatively as 0, 1, or 2 (0, <5%; 1, 5-50%; 2, > or =50%). Expression of CD44 was significantly higher in metastatic RCCs compared to primary RCCs (p = 0.036). CD44 immunoreactivity in the primary RCC (MET- and MET+) correlated with progression-free survival (p = 0.027). In metastatic RCCs, CD44 immunoreactivity also correlated with survival after detection of first metastasis (p = 0.011). In multivariate analysis, stage (p = 0.0001) and CD44 immunoreactivity (p = 0.03) in primary RCC were independent predictors of progression-free survival. Our study suggests that CD44 status in RCC provides useful prognostic information both in primary and metastatic RCCs and may have applicability in stratifying patients for therapeutic decisions.