Disodium cromoglycate has been reported to benefit some patients with active ulcerative colitis. We have investigated the effect of cromoglycate in high dose on rectal mucosal mast cell degranulation (assessed indirectly by measurement of mucosal histamine release, using in vivo rectal dialysis, and the eosinophil content of the rectal inflammatory exudate) and on disease activity in eight patients in relapse. Although cromoglycate (1600 mg by mouth daily for 2 weeks) did not affect histamine release, it reduced (P < 0.05) the eosinophil content of the exudate towards normal. Stool frequency also fell (P < 0.05) but other aspects of bowel habit, sigmoidoscopic appearance, rectal mucosal potential difference and electrolyte transport were unaltered. Pre-treatment values of histamine release and eosinophil exudation could not be used to predict the response to cromoglycate. The reduction in eosinophil exudation produced by the drug could be due to inhibition of mast cell release of chemotactic factors other than histamine; the therapeutic effect of cromoglycate was unimpressive.