Between January 1988 and December 1990, 23 patients with rectal cancer were evaluated by clinical staging (23) and transrectal sonography before (22, one complete stenosis) and after radiotherapy (21). Two criteria were correlated with histological findings: a) the confinement to the rectal wall or spread beyond and b) the presence of meso-rectal lymph node involvement. The 22 patients who underwent transrectal sonography before radiotherapy were staged as uT2: 3 cases, uT3: 17 cases and uT4:2 cases. Fifteen days after irradiation, transrectal sonography showed a regression on the tumor in all cases; the uT stage was different in 9 patients but there was no change in the uN stage. The comparison of the preoperative clinical and sonographic assessment of local invasion beyond the rectal wall with postoperative histopathology revealed a correlation: a) with digital examination in 13 of the 21 patients with palpable tumors, b) with transrectal sonography before irradiation in 12 of the 22 staged tumors, c) with transrectal sonography after irradiation in 17 of the 21 staged tumors. The presence of meso-rectal lymph node involvement determined by histologic examination was correlated with the results of transrectal sonography for 14 of the 21 staged tumors. Transrectal sonography provides a good assessment of the tumor stage before irradiation. This stage should be considered for the study of survival.