IL-8 is generating increasing interest as a powerful neutrophil chemoattractant and activator. To elucidate the mechanisms of neutrophil infiltration in inflammatory bowel disease, we examined 33 patients with ulcerative colitis (UC),18 with Crohn's disease (CD), eight with some other type of colitis, and 18 normal control subjects for measurement of IL-8 in homogenates of colonic biopsy specimens. The affected colonic mucosa was found lo contain significantly more IL-8 in patients with active inflammatory bowel disease than in patients with inactive disease (UC, P<0.001: CD, P<0.001), in patients with other types of colitis (UC, P<0.05; CD, P<0.01), or in normal control subjects (UC. P<0.001; CD. P<0.001). Colonic IL-8 levels correlated significantly with the macroscopic grade of local inflammation, especially in patients with UC (P< 0.001). Colonic lL-8 levels also correlated well with the neutrophil numbers in mucosal tissue (UC, r = 0.950, P<0.001; CD. r=0-940, P<0.001), and with colonic IL-1β (r=0.911, P<0.001) and tumour necrosis factor-alpha (TNF-a) levels (r=0-604. P < 0001) in patients with these two conditions. These data suggest a potential role for IL-8 and its regulatory cytokines IL-I and TNF-a in mediating neutrophil infiltration of the gut wall in inflammatory bowel disease.