The tumor suppressor gene PTEN (phosphatase and tensin homologue deleted from chromosome 10) encodes a dual specific protein and phospholipid phosphatase that affects cell proliferation, apoptosis and migration. In our study, we examined protein expression of PTEN in renal carcinogenesis. PTEN protein levels were examined in 42 clear cell renal cell carcinomas (ccRCC) and oncocytomas as well as in the corresponding normal renal tissue of the same patients using Western blot analysis. Cellular localization was analyzed by immunohistochemistry. PTEN was highly expressed in all investigated normal renal tissue specimens. Immunohistochemical analysis showed an almost exclusive staining of proximal tubulus epithelial cells known to be precursor cells of ccRCC. Within the proximal tubulus cells, PTEN exhibited a membrane predominant immunostaining pattern. In ccRCCs PTEN expression was markedly reduced to an average of less than 10% compared with normal tissue as evidenced by Western blot analysis (p < 0.001). The degree of reduction was similar in highly differentiated (G1) carcinomas and in less differentiated (G2-G4) carcinomas. These observations were reproduced by immunohistochemical studies, which revealed a loss of the characteristic membrane predominant immunostaining pattern in ccRCC. In contrast to the PTEN positive proximal tubulus epithelial cells, the distal tubulus epithelial cells, which are precursor cells of the benign oncocytomas, exhibited only a very weak PTEN expression. Compared with the distal tubulus epithelial cells, no downregulation of PTEN was seen in oncocytomas. We conclude that PTEN expression and PTEN membrane localization are lost during early renal cell carcinogenesis and may therefore be a valuable RCC tumor marker.