Few data exist concerning the ability of intraoperative colonic lavage to decrease colonic bacterial counts, and nothing is known about its effect on the recently described mucosa-associated bacteria. The goal of our study was to determine the impact of intraoperative colonic lavage on both the intraluminal and mucosal microflora. After intraoperative colonic washout in 10 adult male Sprague-Dawley rats, quantitative and qualitative aerobic and anaerobic cultures of the intraluminal and mucosal bacteria were obtained. Tissue was also removed for scanning electron microscopic examination of the colon wall. Whereas 1000-fold to 10 000-fold reductions of aerobic and anaerobic intraluminal flora were achieved with mechanical lavage, reductions of aerobic or anaerobic mucosal bacteria were not significant. Failure to diminish bacterial colonization in this ecologic niche may be partly responsible for the persistently high infection rate after emergency colorectal surgery.