Purpose We evaluated our experience with renal cortical tumors to determine whether tumor size is associated with malignant histology and/or nuclear grade. Materials and Methods We identified 2,675 patients treated surgically at our institution for renal cell carcinoma or a benign tumor between 1989 and 2007. Histological subtype and tumor size were obtained from our kidney cancer database and logistic regression analysis was performed. Results Of the 2,675 tumors 311 (12%) were benign and 2,364 (88%) were renal cell carcinoma. The OR for the association of malignancy with tumor size was 1.16 (95% CI 1.11–1.22, p <0.001), indicating that each 1 cm increase in tumor size was associated with a 16% increase in the odds of malignancy. The incidence of benign tumors decreased from 38% for tumors less than 1 cm to 7% for tumors 7 cm or greater. In patients with clear cell renal cell carcinoma each 1 cm increase in tumor size increased the odds of high grade disease (Fuhrman grade 3–4) compared with low grade disease (Fuhrman grade 1–2) by 25% (OR 1.25, 95% CI 1.21–1.30, p <0.001). In this subset the incidence of high grade lesions increased from 0% for tumors less than 1 cm to 59% for tumors greater than 7 cm. Conclusions Our results confirm previous observations suggesting that the risks of malignancy and high grade tumors increase with tumor size. Patients with small renal masses are at low risk for harboring a high grade clear cell malignancy, which may be useful during initial consultation.