Accurate preoperative staging of rectal cancer is necessary to identify patients who might benefit from adjuvant therapy. Magnetic resonance imaging (MRI) was evaluated in 20 consecutive patients with rectal cancer undergoing 'curative' surgery. Detailed histopathological examination of the resected lesion was correlated with findings of MRI. MRI staging concurred with histological staging in 18 of 20 patients using the Dukes or tumour node metastasis classification but in only 14 using the modified Astler-Coller system. MRI diagnosed transmural invasion in all but one patient with microscopic mural invasion (positive predictive value, 100 per cent; negative predictive value, 80 per cent; overall accuracy, 95 per cent). MRI correctly diagnosed tumour deposits or involved lymph nodes in 12 patients. MRI overstaged one patient, in whom nodes that were enlarged on imaging studies were negative at histological examination (positive predictive value, 92 per cent; negative predictive value, 100 per cent; overall accuracy, 95 per cent). MRI has a role in selected cases for the preoperative assessment of rectal carcinoma.