The relationship between serum angiotensin-converting enzyme activity and inflammatory bowel disease was investigated in 37 patients who had Crohn's disease, 31 patients who had ulcerative colitis, and 104 control subjects. The enzyme activity tended to be depressed in Crohn's ileitis (P less than 0.05) and colitis, but not in Crohn's ileocolitis and ulcerative colitis. No increase in enzyme activity was observed in Crohn's disease ileum or colon or in ulcerative colitis colon. The granulomatous inflammation in Crohn's disease differs from that in sarcoidosis, in which striking elevation of angiotensin-converting enzyme is present in granulomatous tissue and frequently in serum.