['The prevailing concept is that lack of fusion of the anterior and posterior renal fasciae caudally (an open cone) allows free communication between the perirenal space and the extraperitoneal portion of the pelvis. However, perirenal disease rarely extends into the pelvis and an open cone has not been observed on CT scans. Accordingly, we determined the anatomy of the caudal extent of the cone of the renal fasciae in cadavers and on CT scans.', 'Anatomic dissections of the lower portion of the retroperitoneum and the extraperitoneal portion of the pelvis were made in eight cadavers. Two cadavers were intact, two had colored latex injected into the perirenal space before dissections, and the abdomens and pelves of four were sectioned transversely in 3- to 5-cm-thick slices. The renal fasciae were traced on transparent films placed on the cross sections, and computer-generated three-dimensional representations of the tracings were made. These anatomic findings were correlated with observations made on CT scans of 59 consecutive patients with diseases involving the lower part of the retroperitoneum and the extraperitoneal portion of the pelvis (32 patients with hemorrhage, 16 with inflammatory processes, and 11 with neoplastic conditions).', 'The anatomic study showed that the anterior and posterior renal fasciae merge to form a single multilaminar fascia in the iliac fossa. Anteriorly, this common fascia is loosely connected to the parietal peritoneum. Posteriorly lies the caudal continuation of the posterior pararenal compartment. This joins with the laterocaudal continuation of the central part of the retroperitoneum, which contains the iliac vessels. The distal part of the ureter lies within the caudal continuation of the single multilayered renal fascia. The CT studies done in patients showed that extension of the perirenal processes to the pelvis and vice versa was both restrained and uncommon: no direct extension of any abnormalities was observed in either direction, and laminar thickening of the fasciae was seen in one fifth of the patients. Similarly, no inferior communication of the perirenal space with the anterior or posterior pararenal spaces was seen.'] Does why perirenal disease extend into the pelvis : the importance of closure of the cone of the renal fasciae?