The development of a sarcomatoid morphotype is recognized as an extreme form of dedifferentiation in renal cell carcinoma and is associated with a poor prognosis. Although sarcomatoid renal cell carcinoma shows pronounced spindle cell morphology, clear cell renal cell carcinoma may show early spindle cell change with cellular elongation, and the prognostic significance of this is debated. To determine the relationship between sarcomatoid renal cell carcinoma and clear cell renal cell carcinoma showing early spindle cell change, we have investigated collagen expression using immunohistochemistry in these 2 tumor types. Both sarcomatoid renal cell carcinoma and early spindle cell change tumors showed pericellular interstitial expression of collagen types I and III, whereas sarcomatoid renal cell carcinoma also showed cytoplasmic expression of these collagen types. Expression of these collagen types in typical clear cell renal cell carcinoma was, in occasional cases, limited to faint and patchy staining in a pericellular interstitial distribution. Tumor cells did not stain for collagen type IV in sarcomatoid renal cell carcinoma, early spindle cell change, or typical clear cell renal cell carcinoma. In sarcomatoid renal cell carcinoma, there was diffuse pericellular expression of collagen type V and patchy pericellular expression of collagen type VI, whereas early spindle cell change tumors showed patchy pericellular staining with antibodies to collagen type V. Collagen type VI expression in early spindle cell change was largely confined to the vascular adventitia and areas of scarring, although very occasional foci of faint interstitial staining were also seen. In typical clear cell renal cell carcinoma, staining of collagen types V and VI was limited to the vascular adventitia and foci of desmoplasia, whereas no staining of tumor cell cytoplasm were seen. This study has shown that collagen expression of sarcomatoid renal cell carcinoma differs from that of early spindle cell change and provides validating evidence that these 2 morphotypes should not be considered together for classification purposes.