By means of a recently developed isotope washout technique, regional intestinal blood flow and its intramural distribution were determined during surgery and correlated to the morphological inflammatory and vascular features of ulcerative colitis (UC) and Crohn's disease (CD) at different stages and location. In severe colitis total blood flow was significantly increased (64 +/- 9 ml per min X 100 g; mean +/- SE; n = 13) both in UC and CD, the mucosal-submucosal blood flow amounting to 121 +/- 25 ml per min X 100 g (n = 7). The muscularis blood flow was within normal range (18 +/- 10; n = 7), however. In chronic long-standing quiescent or inactive UC, in "healed colitis," and in chronic segmental colitis (CD), colonic blood flow was normal or even reduced (13 +/- 2; n = 8), the decrease in flow comprising both the mucosa-submucosa (21 +/- 3; n = 3) and the muscularis (7 +/- 3; n = 3). In early exudative stage of CD in the ileum total blood flow was normal (26 +/- 5; n = 7) with a normal mucosal-submucosal blood flow (35 +/- 5; n = 3). In late fibrosing stage total blood flow was reduced (11 +/- 1; n = 6) as was the mucosal-submucosal flow (10 +/- 4; n = 3). The muscularis blood flow was reduced in both these stages of ileal CD (7 +/- 1; n = 6). There was a satisfactory agreement between the obtained blood flow figures and the morphologically observed vascular pattern.