A short-term double-blind controlled trial has been performed comparing a slow-release 5'-aminosalicylic acid (5-ASA) tablet (Pentasa) with placebo in the outpatient treatment of mild and moderate active Crohn's colitis. The disease activity was measured objectively by quantitation of the faecal excretion of 111In-labelled autologous granulocytes, which directly reflects gut inflammation. Twelve patients were randomized to receive either Pentasa or placebo. After 10 days of treatment faecal granulocyte excretion fell in all patients receiving 5-ASA, while no consistent change was found in the placebo group. These results suggest that this slow-release preparation of 5-ASA should be tested in larger controlled therapeutic trials over a longer period of time.