Computed Tomography (CT) with rectal air inflation was compared with transrectal ultrasound (TRUS) in the preoperative staging of lower rectal cancer in 126 patients. Precontrast and postcontrast CT scans were performed with 5 mm thick slices; the rectum was previously inflated with air and antiperistaltic agents were administered. Preoperative results were compared with histologic findings. The accuracy, sensitivity and specificity of CT in predicting perirectal spread were 76%, 62% and 83%, whereas the corresponding figures for TRUS were 84%, 69% and 92%. The accuracy, sensitivity and specificity of CT and TRUS for nodal involvement were 58%, 60%, 57% and 72%, 68% and 66%, respectively. These results show that TRUS predicts perirectal spread and detects nodal metastases better than CT. However CT, when performed appropriately, shows tumor spread into perirectal fat and locoregional lymph nodes with high accuracy. Lymphatic involvement is strictly correlated with tumor size: TRUS and CT correctly staged only 57% and 43%, respectively, of the cases with nodal metastases and max. diameter of 5 mm. TRUS sometimes overstaged perirectal tumor growth (13 patients in our series) due to perirectal inflammation (9 cases) or artifacts caused by the presence of air bubbles between the probe and the tumor surface (4 patients). TRUS is a very useful tool for detecting tumor distance from the anal opening; in our series, the distance was incorrectly calculated only in one case (3 cm with TRUS versus 4 cm at surgery).