OBJECTIVE To identify the reasons for the discrepancies between computed tomographic (CT) and pathologic staging of transitional cell carcinoma of the renal pelvis and to develop new criteria to increase the accuracy of CT in staging. METHODS CT scans of 31 consecutive patients with renal pelvic transitional cell carcinoma were evaluated. CT and pathologic staging were compared. RESULTS Pathologic staging revealed four stage 0 tumors, three stage I, five stage II, 10 stage III, and nine stage IV. The initial overall CT staging accuracy was 52% (16 of 31 patients). The sensitivity for minimal invasion was 17% (two of 12 patients). Two-thirds (10 of 15 patients) of the misinterpreted cases were overstaged as stage III. Proximal hydronephrosis was present in 80% of overstaged cases (eight of 10 patients). Reevaluation of the CT studies by using proximal hydronephrosis as a criterion for minimal invasion improved overall CT staging accuracy (77%). The revised staging yielded a sensitivity of 83% and specificity of 95% for minimal invasion and improved the specificity for deep invasion (17% to 92%). CONCLUSIONS In a patient with transitional cell carcinoma of the renal pelvis, hydronephrosis proximal to the tumor may cause overstaging of stage 0-II disease and may not indicate more advanced disease.