We evaluated the efficacy of an oral formulation of 5-amino-salicylic acid in lowering the relapse rate after remission of Crohn's disease. Included were 59 patients who had proven Crohn's disease of at least 1 year's duration, and who had been in continuous remission for at least 6 months, while taking only 5-aminosalicylic acid or no therapy at all. Remission was defined as a Harvey Bradshaw index score (Softley-Clamp modification) of < 4. Patients were given coded mesalzaine 250 mg or placebo tablets (2 x 2 day). They were seen at 0, 1, and 2 months, and then every 2 months until the end of the study. Trial endpoints were 1 year of follow-up, or clinical relapse results. After randomization, 31 patients were included in the placebo arm, and 28 in the treatment arm. There were no significant differences between the two groups at entry. Ten patients were withdrawn from the trial because of noncompliance, loss of follow-up, or headache. There were more clinical relapses in the placebo arm (15 patients, 55%) than in the treatment arm (6 patients, 27%) (p < 0.05). Mesalazine had a significant advantage over placebo (p < 0.05) only in the subgroups of patients with ileal Crohn's disease and in those older than 30 years. We conclude that mesalazine has a moderate but significant benefit in preventing relapse in Crohn's disease in remission; this occurred only in patients with small-bowel involvement or in those older than 30 years.