Transrectal ultrasonography of rectal carcinoma. Preoperative transrectal ultrasonographic staging of 38 patients with known adenocarcinoma of the rectum was performed. Six of these patients had preoperative radiation therapy. Of the 32 without radiation therapy, transrectal ultrasonography (TRUS) correctly staged disease limited to the rectum in 90% (18 of 20) and transmural extension in 50% (6 of 12), for a sensitivity for evaluating transmural extension of 50%, specificity of 90%, positive predictive value of 75%, and a negative predictive value of 25%. Nineteen patients had positive nodes. The sensitivity and specificity of detecting nodal metastases by TRUS were 63% and 85%. The positive and negative predictive values were 86% and 61%, respectively. TRUS was slightly superior to digital rectal examination in staging patients because of its superior ability to detect adenopathy. Adjacent organ involvement was correctly evaluated in five of six patients; peritumoral fibrosis was misinterpreted as tumor extension into uterus and ovary in one woman. Tumor shrinkage in all patients undergoing radiation therapy was identified, but fibrosis and edema made staging by TRUS unreliable in those patients. 