histologic subtype and nuclear grade, and illustrate the role of renal biopsy in surgical versus non-surgical management. METHODS: A retrospective chart review was performed including patients in the Southern California Kaiser Permanente Medical Region that underwent either CT or ultrasound guided core percutaneous renal biopsy of a solid renal mass from January 2005 to December 2009. Patients were stratified by size of renal mass, comparing SRMs (less than or equal to 4 cm) to larger renal tumors. Initial biopsy results including tumor histology and nuclear grade were evaluated and related to postoperative pathology specimens when surgery was performed. RESULTS: The study included 126 patients (129 renal units with 132 biopsies). Sixty-three patients (50%) ultimately underwent surgery (23 partial and 40 radical nephrectomies). Patients who did not have surgical intervention tended to be older, average 68 years old versus 61 years old and have larger tumors 5.5 cm versus 4.0 cm. The overall sensitivity for detecting cancer (verified by final pathology) in our series was 75.4% with 100% specificity. The positive predictive value for patients who underwent surgery was 100% and negative predictive value 11.7%. When evaluating factors associated with accuracy of predicting cancer, larger tumor size has an odds ratio of 1.21 (p 0.10) and increasing number of biopsy samples has an odds ratio of 1.57 (p 0.17). CONCLUSIONS: Percutaneous renal needle biopsy in the modern era has respectable sensitivity, excellent specificity and good concordance with final pathology. This modality can assist in clinical decision-making for renal masses, particularly for SRMs, as treatment options are expanding.