Olsalazine is a compound consisting of two 5-amino salicylate (5-ASA) molecules linked by an azo bond, which, administered orally, is split by colonic bacteria to liberate 5-ASA. It lacks the sulfapyridine moiety found in sulfasalazine. Using a specific protocol, we conducted a randomized, double-blind, placebo-controlled trial of olsalazine in patients with symptomatic ulcerative colitis. Inclusion criteria included mild to moderate disease with involvement of more than 15 cm of colon, visible blood in stools, and the discontinuation of all other medications prior to and during the study. Patients were given oral olsalazine 3.0 g/day or placebo for 4 wk. Patients were evaluated clinically, by laboratory analysis and by colonoscopic evaluation, at entry and at 4 wk. Additional clinical and laboratory evaluations were performed at 2 wk. Fifteen patients entered the study. Of the seven patients randomized to olsalazine, four (57%) improved clinically and by colonoscopic scoring, one showed no improvement in either, and two (29%) withdrew after developing severe watery diarrhea. Of the eight patients treated with placebo, two (25%) improved clinically but were without colonscopic improvement and six (75%) worsened, of whom four withdrew early because of worsening symptoms of colitis. Seven of eight placebo patients were then treated with olsalazine on an open basis. Of these seven, five (71%) improved clinically and colonoscopically and two (29%) withdrew because of severe watery diarrhea. Overall, of 14 patients treated with Olsalazine, nine (64%) improved, one showed no improvement, and four (29%) discontinued because of persistent watery diarrhea. No other serious side effects were noted. Minor side effects included transient diarrhea, flares of acne, and anxiety attacks which resolved despite continuation of the drug.