In this study the value of endorectal ultrasound in diagnosis and therapy of large rectal adenomas is shown. Between January 1988 and August 1992 93 patients with the primary diagnosis of adenoma proven by biopsy were examined by endorectal ultrasound. Sonographical differentiation of adenoma and carcinoma was done by echo-structure. Enlarged lymph nodes can be detected too. In 34 of 91 patients carcinoma was found by endorectal ultrasound and 28 were verified histologically. Sensitivity of endorectal ultrasound in detection of carcinoma in enlarged adenomas is 96%. Only preoperative endorectal ultrasound could give decisive criteria to take a second biopsy, to avoid mucosectomy in case of a malignoma or give the advice for a radical operation according to the tumor stage. Further staging of adenomas and small carcinomas is excellent by endorectal ultrasound and the right indication of local therapy can be confirmed.