Collagenous colitis (CC) is an inflammatory bowel disease of unknown aetiology and pathogenesis. In ulcerative colitis and Crohn's disease, prostaglandins may be involved in the pathogenesis of inflammation, and increased expression of cyclo-oxygenase-2 (COX-2) has been detected. The purpose of this study was to examine the presence and cellular localization of COX-2 in colonic mucosa of patients with CC. Using immunohistochemistry, immunoflouresence and Western blot analysis, COX-2 expression was evaluated in colonic mucosal biopsies from 10 patients with active untreated CC, and compared with samples from eight normal controls, and samples from eight patients with ulcerative colitis or Crohn's disease. Specimens from patients with CC expressed COX-2 protein in increased amounts compared with controls, but similar to patients with ulcerative colitis and Crohn's disease. COX-2 expression was localized to the mononuclear cells of the lamina propria. COX-2 expression was most evident in macrophages. Co-localization of COX-2 and macrophages was increased in number in comparison with controls. In conclusion COX-2 is expressed in increased amounts primarily in the macrophage subpopulation of the inflammatory infiltrate of lamina propria in CC. Increased recruitment of macrophages, increased expression of COX-2 and increased prostaglandin synthesis may be involved in the pathogenesis of CC.