Our work was aimed at assessing the accuracy of CT and MRI in the early identification of postoperative recurrences of rectosigmoid cancer, quantifying false positive and false negative ratios and questionable findings. A homogeneous series of 50 patients submitted to surgery for primary rectosigmoid carcinoma was studied with both CT and MRI and followed-up for up to 2 years: local recurrences were observed in 15 patients (30%) which occurred within the first year of surgery in 67% of cases and were in extraluminal site in 86% of cases. CT appeared to be a reliable and highly sensitive screening method, with 82% sensitivity: only 2 false negatives were observed, which were nevertheless followed-up because CT had yielded questionable, and not negative, findings. In contrast, CT had only 78% specificity, being unable to differentiate fibrosis, displaced normal structures and recurrences in 7 cases; its positive predictive value (PPV) was 66%, with 89% negative predictive value (NPV) and 80% accuracy. MRI had a complementary role to CT, because of its capabilities in discriminating all the questionable CT cases and in identifying all CT false positives, thanks to its higher specificity (100%). MRI had 74% sensitivity, which was lower than that of CT; MRI yielded 2 false negatives which, however, had been previously diagnosed with CT. MRI had 100% PPV, 89% NPV and 92% accuracy. Therefore, for the early detection of rectal cancer recurrences, the following diagnostic protocol is suggested: CT should be performed first, as a screening method, within 2-4 months of surgery, and repeated every 6-8 months during the first 2 years-together with CEA values monitoring. MRI should be reserved to the patients in whom CT findings were positive, questionable, or in disagreement with clinical symptoms and/or with increasing CEA values. If MRI fails to solve the diagnostic doubt, a CT-guided biopsy of the mass should be performed.