The records of 13 patients with symptomatic rectal obstruction secondary to prostatic carcinoma have been critically reviewed to provide criteria for identifying this atypical presentation. The site of obstruction varied from the anal verge to 17 cm by sigmoidoscopy. Significant clinical findings were ureteral obstruction on excretory urography in 12 of the patients, elevated acid phosphatase in eight, intact mucosa over the obstructing mass in seven and osteoblastic bone metastasis in six. Primary diagnosis was generally established by comparing histopathologically the prostate and the obstructing lesion. Colostomy was necessary in nine cases. In three patients colostomy was obviated in partially obstructing lesions by intravenous diethylstilbestrol diphosphate therapy. One patient had an inappropriate low anterior resection. Rectal obstruction is most commonly due to primary adenocarcinoma. Occasionally the obstruction is secondary to prostatic carcinoma masquerading as an intrinsic rectal cancer. Identification of the true nature of the illness is essential for proper treatment.