Between January 1988 and January 1992, 34 patients with rectal cancer were evaluated both by clinical examination and endosonography before and after pre-operative radiotherapy. Two criteria were correlated with histologic findings: confinement to the rectal wall or spread beyond, the presence of mesorectal lymph node involvement. The 32 patients who underwent endosonography before radiotherapy were staged as: uT2: 4, uT3: 26, uT4: 2 cases. Fifteen days after irradiation, endosonography showed tumour regression in all cases; uT stage was different in 15 patients, uN stage in 4 cases. Comparison of the pre-operative local invasion beyond the rectal wall with postoperative histopathy revealed a correlation with: digital examination after radiotherapy in 20 of the 31 patients with palpable tumours; endosonography before irradiation in 18 of the 32 staged tumours; endosonography after irradiation in 25 of the 32 staged tumours. The presence of mesorectal lymph node involvement determined by histologic examination was correlated with the results of endosonography after radiotherapy for 22 of the 32 staged tumours. Endosonography provides a good assessment of the tumour stage before irradiation. Since radiotherapy alters endosonographic staging of rectal cancer, this staging should be included in survival studies.