Renal sinus fat invasion was incorporated as one of the parameters for pT stage definition of renal cell carcinoma (RCC) only in the latest 2002 American Joint Committee on Cancer/tumor-node-metastasis staging protocol. The current pT3a subcategory (in addition to adrenal gland involvement) groups 2 modes of extrarenal extension by RCC, either by peripheral perinephric fat extension or by renal sinus fat invasion. Recent prospective studies have shown that with more directed gross sampling and histologic evaluation, renal sinus invasion is actually more commonly diagnosed than previously reported, or when compared with retrospectively sampled RCC nephrectomy specimens. These studies have demonstrated that renal sinus invasion is the principal pathway for extrarenal extension for clear cell RCC; the incidence of which is related to size (tumors greater than 4 cm more frequently involve the renal sinus). More significantly, a recent retrospective study of pT3a clear cell RCC nephrectomy specimens showed that tumors invading the renal sinus fat portend a more aggressive outcome than tumors invading only the peripheral perinephric fat. Clear cell RCCs invading the renal sinus are more likely to have higher nuclear grade, regional lymph node involvement and sarcomatoid transformation than tumors invading only the perinephric fat. Given the importance of renal sinus invasion, sampling strategies for nephrectomy specimens should be modified to focus in this region as appropriate and pathologists should be familiar with the histologic criteria for staging renal sinus invasion.