The effects of sphincter-saving resections for carcinoma of the rectum on bladder function were studied prospectively. Twenty-seven patients, each acting as his or her own control, were studied before, shortly after and 9 months after operation by means of pressure/flow filling and voiding cystometry. After operation there was a significant and lasting increase in the residual volume of urine and a temporary decrease in the compliance of the bladder. There was a statistically significant decrease in detrusor contraction pressure after operation, which persisted throughout the period of study. This was probably due to partial denervation of the bladder. Four patients had signs of total or almost total denervation of the bladder. There was a significant correlation between proximity of the tumour to the anal verge and risk of damage to the nerve supply to the bladder. Thus sphincter-saving resections of the rectum for carcinoma are associated with a significant risk of bladder denervation. Many of the "minor" symptoms of bladder dysfunction which develop after this procedure are due to partial denervation of the bladder.