One hundred sixty patients intolerant of or allergic to sulfasalazine (Salazopyrin, Azulfidine) participated in an open tolerance study of azodisal sodium (Dipentum). More than 4 of every 5 patients tolerated azodisal sodium well, but 12.5% of patients stopped medication because of diarrhea. Even after 7 patients who had also experienced diarrhea when taking sulfasalazine were excluded, there still remained a group of patients (9.8%) who had to discontinue azodisal sodium because of diarrhea. Apart from this, only minor side effects occurred. No serious drug-related changes were seen in hematologic or biochemical parameters. Male fertility appeared to be unaffected. One hundred two patients, who were in clinical and sigmoidoscopic remission, took part in a double-blind, placebo-controlled maintenance trial. Of these, 23.1% of the patients treated with azodisal sodium and 44.9% of the patients treated with placebo had a clinical and sigmoidoscopic relapse during a 6-mo trial period (p = 0.02). Azodisal sodium appears to be an effective agent for the maintenance treatment of ulcerative colitis.