One hundred patients with rectal adenocarcinoma were examined preoperatively with rectal endosonography (ES) and 50 were also examined with computed tomography (CT). ES predicted mesorectal lymph node involvement with an accuracy of 83 per cent, sensitivity of 88 per cent, specificity of 79 per cent, positive predictive value of 78 per cent and negative predictive value of 89 per cent. CT in comparison had an accuracy of 57 per cent, sensitivity of 25 per cent, specificity of 91 per cent, positive predictive value of 75 per cent and negative predictive value of 53 per cent. No particular histological architectural feature could be identified as responsible for false positive diagnosis though nodal size was significantly larger in the true positive and false positive group when compared with the true negatives (P less than 0.001 and P less than 0.01 respectively).