The lymphatic drainage of diseased and normal bowel was studied in 21 patients undergoing surgery for Crohn's disease. Mesenteric lymphatic obstruction was a consistent feature, identified in areas of small bowel macroscopically affected by Crohn's disease. This finding was also observed in some areas of apparently unaffected small bowel. Subsequent examination of these areas confirmed the presence of early Crohn's disease. This method of study may prove to be of value for determining the extent of operative resection in patients with Crohn's disease. Experimental lymphatic obstruction in animals failed to produce pathological changes of Crohn's disease, suggesting that this feature is an epiphenomenon and is not of primary importance in the pathogenesis of regional enteritis.