CT scan imaging was used to explore 215 cases of bladder tumor (48 A-B1, 55 B2, 85 C, 27 D), and findings compared with those at operation and on histopathology of specimens after total cystectomy (141 cases), partial cystectomy (13 cases) or endoscopic resection (61 cases). The radiologic technique used was that of natural contrast, evaluating the negative contrast of intravesical urine in relation to the normal or pathologic bladder wall. Analysis of scan sections, assisted by a previous pathologic and radiologic study of 14 transverse sections of pelvis, allowed definition of a certain number of semiologic criteria corresponding to the various stages of the disease. Results showed global reliability to be relatively satisfactory for assessment of local and regional extension (85%): it was obviously a more effective investigation for tumors of low invasion (A-B1: 75%, B2: 81%) than for widely infiltrating tumors (C: 94%, D: 93%). Glandular extension was less reliably evaluated, however, since global efficacy was only 65%. Failures observed can be explained by the frequent small size of the metastatic glands, the poorness in cellular and fatty tissues and, particularly, by the fact that the scanner provides data of size of glands only and not on their internal architecture. The place of the scanner among other paraclinical methods of evaluating tumoral extension has still to be determined.