Preoperative external beam radiation therapy (RT) is an increasingly popular form of adjunct therapy for rectal cancer; however, little is known about its effects on healing of colorectal anastomoses. We serially evaluated colorectal anastomotic healing following moderate dose RT. Twenty-four adult female pigs were administered 4250 centiGrays (cGy) of external beam pelvic RT over 4 weeks followed by rectosigmoid resection with handsewn or circular stapled anastomosis. Groups of animals were then sacrificed at 5, 11, 60, and 120 days postoperatively. A control group (N = 24) was studied similarly but not irradiated. Perianastomotic blood flow was measured using laser doppler velocimetry at each time interval. Nonirradiated anastomoses demonstrated significantly higher blood flows compared to irradiated anastomotic segments at each study interval. In addition, early (Days 5 and 11) postoperative gross inflammatory scores and microscopic inflammatory score (all intervals) were significantly higher for irradiated animals. Anastomotic complications were more frequent in irradiated animals ( 7 24) vs controls ( 2 24, P < 0.05). These results indicate that a preoperative dose of 4250 cGy results in an early and persistent decrease in colorectal mural blood flow independent of anastomotic technique. This is the first experimental study which attempts to quantitate the effects of preoperative pelvic radiation therapy on anastomotic blood flow. © 1992.