In 48 patients who had undergone anterior resection for rectal cancer with straight colorectal reconstruction, clinical and manometric results were correlated with the level of anastomosis. Patients were divided into four groups by anastomotic level: < or = 3, 4-6, 7-9, and > or = 10 cm. Functional outcome with regard to frequency of bowel movements, minor leakage, fecal incontinence. ability to defer stool and to differentiate consistency showed increasing impairment the lower the anastomotic level. Frequency, leakage owing to the inability to defer stool, incontinence for solid stool, inability to discriminate flatus from stool, and incomplete emptying were significantly different (P < 0.05) between the patients with an anastomotic level between 3-6 cm and between 7-9 cm. Manometric data revealed no trend or significant differences among the groups with regard to anal resting pressure and maximal and median squeeze pressure. Rectoanal inhibitory reflex was abolished in 60% of the patients. Clear changes, with a trend toward reduced function with lower anastomotic levels, were seen in the volume that produced a feeling of urgency, maximal tolerable volume, and neorectal compliance (between anastomotic levels 7-9 and > or = 10 cm the differences were significant; P < 0.05). Analysis by length of residual rectum (< 1.5, 1.5-4.0, 4.1-6.5, > 6.5 cm) demonstrated similar findings, suggesting that impaired function after rectal resection is due to reduced function of the neorectum. Thus, as much residual rectum as possible should be preserve without risking cure. If the level of the anastomosis is expected to be below 6 cm, or if the residual rectum is less than 4 cm, the construction of a colon pouch to increase neorectal capacity should be considered.