Sixty-six outpatients with active ulcerative colitis who were intolerant of sulphasalazine were treated in a double-blind randomized trial with placebo or olsalazine in daily doses of 0.75, 1.5, or 3 g. Overall, 35% of patients receiving olsalazine improved clinically, compared to 16% of patients receiving placebo. Statistically significant or nearly significant improvement was demonstrated in colitis activity by the end compared to the beginning of the study within the combined olsalazine group and within patient groups receiving olsalazine at daily doses of 1.5 g and 3 g. There were no differences between the treatment and placebo groups for any of the reported adverse effects or laboratory variables. The data suggest that olsalazine is effective for the treatment of ulcerative colitis and is well tolerated among patients intolerant to sulphasalazine.