: Olsalazine, consisting of two salicylate radicals linked by an azo-bond, is effective in the treatment of active ulcerative colitis. To test its effect in patients with mild to moderate attacks of Crohn's disease, the International Organization for the Study of Inflammatory Bowel Disease (IOIBD) designed a multicentre, randomised, double-blind, placebo-controlled study. Ninety-one patients from four centres were randomised to receive either olsalazine, 1 g b.i.d., or matching placebo tablets. Twenty-six patients had ileal disease; 43, ileocolonic; and 22, colonic. Thirty-five of 46 patients taking olsalazine and 24 of 45 patients taking placebo were withdrawn before the end of the 4-month study. Diarrhoea was the most common reason for withdrawal from the olsalazine group, accounting for 22% of the patients, as compared with 4% in the placebo group. No other side effects were reported. There was no difference in the remission rate or withdrawal rate for active disease in the two groups. However, when an intent-to-treat analysis was performed, only eight of the 46 (17%) olsalazine-treated patients were considered to have entered remission or improved their symptoms compared with 22 of the 45 (49%) placebo-treated patients (p < 0.03). This study was unable to show that patients with mild to moderate attacks of Crohn's disease were significantly improved by treatment with olsalazine at a dose of 1 g daily. However, the potential benefit of a higher dose cannot be excluded.