PURPOSE: To assess the usefulness of CT scanning in the differentiation of stage T3a from T2 in renal cell carcinoma. MATERIALS AND METHODS: Among patients with pathologically proven renal cell carcinoma, 114 at stages T2 and T3a were divided into three groups, as follows: intact capsule (T2) n=40, capsular involvement (T2) n=38, and capsular penetration (T3a) n=36. By referring to contrast-enhanced CT scans, we retrospectively compared the groups in terms of tumor margin, the frequency with which a tumor bulged more than 3 cm beyond the renal contour, the presence or absence of peritumoral collateral vessels, thickening of Gerota 's fascia, and perinephric strands. RESULTS: An irregular margin was more common in the capsular penetration group than in the other two groups (p<0.05). With regard to frequency of tumor bulging, the presence of peritumoral collateral vessels, thickening of Gerota 's fascia, and perinephric strands, these characteristics were more frequently noted in the capsular penetration group (T3a) and capsular involvement group (T2) (p<0.05) than in the intact capsule group. The difference between the capsular penetration group (T3a) and the capsular involvement group (T2) was not significant, however (p>0.05). CONCLUSION: In determining the tumor stage of renal cell carcinoma, CT is not helpful in differentiating between a tumor with capsular penetration (T3a) and one with capsular invasion (T2), though differentiation of the T3a stage from the T2 stage, without capsular invasion, is reliable. When a tumor has an irregular margin, however, the possibility that it is at stage T3a should be considered.