It has been suggested that granulomatous vasculitis is a primary mechanism in the production of pathologic changes seen in Crohn's disease. We set out to investigate the relationship of granulomas to blood vessels and to confirm or refute previous reports of granulomatous vasculitis in Crohn's disease. Thirty paraffin embedded tissues from 11 patients with Crohn's disease were selected after examination of H&E stained sections for the presence of granulomas. Using an immunohistochemical method, various monoclonal antibodies were applied to sequential sections from each tissue to demonstrate vascular structures and granulomas. In three patients none of the granulomas occurred in association with blood vessels, in five a small proportion of the granulomas affected blood vessels, and in three granulomatous vasculitis appeared occlusive and significant. A total of 232 granulomas were identified, 22% of which were closely associated with blood vessels, which included both arteries and veins; 16% were perivascular, while 6% were intravascular. Perivascular granulomas did not surround blood vessels or invade the medial layers. They were asymmetric, suggesting that they originated by encroachment of nearby lymphatic or connective tissue granulomas. These results indicate that the granulomas of Crohn's disease are usually not associated with blood vessels; however, there is a minority of patients in whom vascular granulomatous inflammation may be important, although probably as a secondary phenomenon.