Sixty-six outpatients with active ulcerative colitis who were intolerant of sulfasalazine were treated in a double-blind randomized trial. They received placebo or olsalazine sodium 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. When the colitis activity at study entry was compared with that observed at the completion of the study period, statistically significant or nearly significant improvement was demonstrated within the combined olsalazine group (p = 0.01) and within patient groups receiving olsalazine at daily doses of 1.5 g (p = 0.04) and 3 g (p = 0.055). A dose-response relationship was suggested because 16%, 29%, 27%, and 50% of patients improved in the placebo and 0.75-, 1.5-, and 3-g olsalazine groups, respectively, (p = 0.04). A similar pattern of improvement was seen when sigmoidoscopic criteria were used, although a dose-response relationship was not demonstrated. There were no differences between the treatment and placebo groups for any of the adverse effects or laboratory variables reported at baseline or during the trial period. Four patients were withdrawn because of adverse reactions: 2 developed a skin rash while receiving olsalazine and 2 had diarrhea, one while on olsalazine and the other while on placebo. The data suggest that olsalazine is effective for the treatment of ulcerative colitis and is well tolerated among patients intolerant to sulfasalazine.