Presacral or pelvic floor tumour recurrence after abdomino-perineal resection for rectal or anal carcinomas has not been amenable to early radiological diagnosis. Computer tomography is a procedure which can demonstrate recurrences of two to three cm. Within the pelvis with certainty. The reliability of the method has been demonstrated in 27 patients suspected of having recurrences. The effect of demonstrating or excluding a recurrence on subsequent treatment is discussed. The value of computer tomographic examination of the pelvis for demonstrating the position and extent of a recurrence, the displacement of neighbouring organs, destruction of the pelvis and involvement of iliac lymph nodes is shown.