Objectives: To investigate the relationship between the radiographic size on computed tomography and the pathologic size of renal tumors. Methods: The records of 126 patients with a renal lesion suspicious for malignancy and with preoperative computed tomography performed 60 days or less before surgery at our institution were reviewed. The clinical size was defined as the largest diameter of the tumor seen on computed tomography and the pathologic size was defined as the largest diameter seen at pathology. The clinical and pathologic sizes were compared by size range and primary tumor stage. Results: A total of 133 tumors from 126 patients were identified. Of the 133 tumors, 120 (90.2%) were renal carcinoma. The clinical and pathologic size for all 133 tumors was not significantly different (4.5 versus 4.1 cm, P = 0.35). The average clinical tumor size was larger than the pathologic tumor size for all sizes, except for 7 cm and greater. The difference reached statistical significance in all ranges from 1 to 5 cm. The largest size difference was seen for tumors 4 to 5 cm, for which the average clinical size was 0.87 cm larger than the average pathologic size (P = 0.025). Conclusions: Preoperative computed tomography imaging may slightly overestimate the pathologic size of renal tumors in certain size ranges. In pathologic Stage T1a tumors, the clinical tumor size was significantly larger than the pathologic stage (P = 0.009). The difference between the clinical and pathologic tumor size was greatest for tumors 4 to 5 cm. These results may affect decisions to perform nephron-sparing surgery in certain patients.