The in vitro synthesis of leukotriene B4 (LTB4) was evaluated in colorectal biopsy specimens and resection tissue from patients with inflammatory bowel disease. The in vitro formation of LTB4 from biopsy tissues stimulated with calcium ionophore A23187 correlated with the degree of mucosal inflammation assessed at sigmoidoscopy, and with neutrophil infiltration measured as myeloperoxidase activity. Biopsy specimens from patients taking prednisolone formed less LTB4 than those from patients not on prednisolone, with comparable levels of inflammation seen at sigmoidoscopy. The formation of LTB4 was reduced dose-dependently by the acetohydroxamic acid 5-lipoxygenase inhibitor BWA4C, with no significant inhibition of prostaglandin E2 or thromboxane B2 synthesis. In inflamed colonic resection tissue from colitic patients, the IC50 for inhibition of LTB4 formation by BWA4C was 0.03 mumol/l, compared with an IC50 of 0.8 mumol/l for NDGA. Thus, BWA4C is a potent and selective inhibitor of LTB4 synthesis in colonic tissue from patients with ulcerative colitis. Acetohydroxamic acid 5-lipoxygenase inhibitors, exemplified by BWA4C, may be useful to evaluate the clinical importance of LTB4 in ulcerative colitis, and offer a novel therapy for the disease.