CT scans were carried out on 25 patients with transitional cell carcinoma of the renal pelvis. Of the 25 patients, tumors were identified in 24 patients (96%) and not in one patient on CT scan. Of the 24 patients the tumor was delineated as a solid mass in the renal pelvis and/or calyx in 15 and as an infiltrating mass in the renal parenchyma in 8 on CT scan. The depth of invasion was correctly estimated by CT in 18 of the 25 patients (72%). Whereas the tunica muscularis of the renal pelvis or the renal parenchyma was found involved in 3 of 10 patients (30%) in whom the diagnosis was made that the tumor was limited to the renal pelvic mucosa, the correct diagnosis was possible in 22 of 25 patients (88%) in whom the tumor was confined to the renal pelvic wall (pTa-pT2) or more invasive (pT3-pT4). In 6 of 7 patients with lymph nodes matastases enlarged lymph nodes were seen on the CT scan. In all 7 cases the primary tumor was classified as a pT3 or pT4 invasive disease. Based on the results presented above, it may be concluded that CT scan is valuable in making the diagnosis of transitional cell carcinoma of the renal pelvis and also in determining whether the tumor has invaded beyond the renal pelvic wall, thereby providing guidelines for the adequate treatment.