Piroxicam decreases postirradiation colonic neoplasia in the rat. This study evaluated the effects of the nonsteroidal antiinflammatory agent piroxicam on chronic radiation proctitis in the rat. Forty female Wistar rats received a 2250-cGy dose of irradiation to the distal 2 cm of the colon. Twenty received piroxicam 8.0 mg/kg orally 30 minutes before exposure and 24 hours after exposure; 20 rats served as irradiated controls. All animals were evaluated by colonoscopy 1 and 3 weeks postexposure and every third week until death or killing at 1 year. At killing, colons were removed for light microscopic examination. One year postirradiation results showed no differences in mortality, vascular changes, acute inflammation, colitis cystica profunda, or rectal stricture between the control and piroxicam-treated groups. However, at 1 year postirradiation the control group demonstrated neoplasia in 15 of 19 animals compared with eight of 20 animals in the piroxicam-treated group. The first endoscopic appearance of colonic neoplasm occurred at 15 weeks postirradiation in one control irradiated rat whereas the first evidence of endoscopic neoplasm in the piroxicam-treated group did not occur until 36 weeks postirradiation. Histologic examination documented a tendency toward a greater presence of adenocarcinomas in the control group compared with the piroxicam-treated group. The authors conclude that piroxicam treatment significantly decreased the incidence of colonic neoplasia in general as well as delayed the endoscopic appearance of colonic neoplasia in rats after pelvic irradiation. 