BACKGROUND AND AIMS: The benefit of 5-aminosalicylic acid therapy for maintenance of remission in Crohn's disease is controversial. The primary aim of this study was to evaluate the prophylactic properties of olsalazine in comparison with placebo for maintenance of remission in quiescent Crohn's colitis and/or ileocolitis. METHODS: In this randomised, double blind, parallel group study of olsalazine versus placebo, 328 patients with quiescent Crohn's colitis and/or ileocolitis were recruited. Treatment consisted of olsalazine 2.0 g daily or placebo for 52 weeks. The primary end point of efficacy was relapse, as defined by the Crohn's disease activity index (CDAI) and by clinical relapse. Laboratory and clinical disease activity indicators were also measured. Safety analysis consisted of documentation of adverse events and laboratory values. RESULTS: No differences in the frequency of termination due to relapse or time to termination due to relapse were noted between the two treatment groups (olsalazine 48.5% v placebo 45%) for either colitis or ileocolitis. The failure rate, defined as not completing the study, was significantly higher in olsalazine treated patients compared with placebo treated patients for the overall population (colitis and/or ileocolitis: olsalazine 65.4% v 53.9%; p=0.038). Similar failure rates were seen for patients with colitis. A significantly higher percentage of olsalazine treated patients experienced adverse gastrointestinal events. Drug attributed adverse events were reported more frequently in the olsalazine treated group with gastrointestinal symptoms being causally related to olsalazine treatment (olsalazine 40.7% v placebo 26.9%; p=0.010). Back pain was reported significantly more often by the placebo treated group. However, serious medical events did not differ between the two groups. Adverse events led to more early withdrawals in the olsalazine treated group than in the placebo treated group; thus average time in the study for patients in the olsalazine treatment group was significantly shorter than that of patients in the placebo group. CONCLUSIONS: Patients treated with olsalazine were more likely to terminate their participation in the trial than those taking placebo. This difference was not related to relapse of disease, as measured by CDAI and clinical measures, but rather was due to the development of intolerable adverse medical events of a non-serious nature related to the gastrointestinal tract. The gastrointestinal related events in the olsalazine treated group may be due to the difference in gastrointestinal status at baseline which favoured the placebo treatment group.