In five cases of Whipple's disease the vessels in several organs of the gastrointestinal system were carefully examined for PAS-positive bacilli. Bacilli were particularly abundant in the arteries of the small intestinal serosa and the liver, but they were also present in stomach, colon, gallbladder and mesenteric lymph nodes. The predominant site of involvement was the arterial tunica media. In addition to focal degeneration and fibrosis in the tunica media, other local reactions included arteritis and intimal proliferation. Within the tunica media and less often the endothelium the bacilli were found alone or in small clumps, but in the tunica adventitia they were usually present within macrophages. The possible significance of this Whipple's arteriopathy is considered relative to unexplained gastrointestinal bleeding, focal ischemia due to arterial narrowing, and some role in the further dissemination of bacilli being shed from the endothelium.