Purpose: We determined the safety and accuracy of preoperative needle core biopsy for diagnosing benign lesions among small incidental asymptomatic renal masses.Materials and Methods: Between February 2000 and December 2007 we received a total of 235 preoperative core biopsies from 222 less than 5 cm incidental renal masses. Biopsy results were correlated with surgical specimen final pathology findings or with patient followup if surgery was avoided.Results: Of the 235 biopsies 184 (78%) were diagnostic, whereas 51 (22%) were nondiagnostic due to insufficient material or contents of only normal, inflammatory, fibrotic or necrotic tissue, or blood clot. Diagnostic biopsies revealed 138 malignant (75%) and 46 benign (25%) lesions. Of these patients 108 (59%) underwent renal surgery, which showed a 100% biopsy accuracy rate for distinguishing malignant from benign lesions and a 98% rate for determining histological tumor type. Followup with radiological imaging was performed for 59 lesions in patients with nondiagnostic biopsies or benign masses and for 16 low grade malignant tumors in elderly patients. Lesions remained stable in 61 cases, showed minor size changes in 9 and resolved in 5. No patient has shown symptoms or required renal surgery to date. Significant biopsy related complications were noted in only 2 patients (0.9%).Conclusions: We found that needle core biopsy was a safe and accurate technique for distinguishing between malignant and benign tumors in small asymptomatic incidentally detected renal masses. Biopsy of small tumors is associated with a relatively high rate of technical biopsy failure, although this may be addressed by adopting improved biopsy techniques, as discussed.