Corticosteroid treatment reduces mast cell numbers in inflammatory bowel disease. Mast cell degranulation in the gut causes mucus secretion, mucosal edema, and increased gut permeability and may be responsible for some of the symptoms and signs of inflammatory bowel disease. We have used a novel monoclonal antibody (AAI) against tryptase expressed exclusively in the granules of mast cells to enumerate mast cells in rectal biopsies in order to study the effect of inflammatory bowel disease and drug treatment upon rectal mast cell numbers. Rectal mast cell numbers are significantly reduced in inflammatory bowel disease patients taking corticosteroids (mean 4.95 cells/mm2) when compared with control patients (10.1, P less than 0.001) and inflammatory bowel disease patients not taking corticosteroids (9.7, P less than 0.001 Wilcoxon rank sum test). The reduction in mast cell counts was independent of the degree of inflammation or architectural distortion. There was a negative correlation between the dose of corticosteroids and mast cell count (r = 0.53, P less than 0.05 Spearman rank correlation), and the mast cell count was reduced within a few days of treatment and remained low throughout steroid therapy. Mucosal mast cell depletion may be an important mechanism of action of corticosteroids in inflammatory bowel disease. 