A multicentre double-blind study was conducted to evaluate the efficacy and tolerability of 1 g or 1.5 g mesalazine daily compared with placebo in 94 patients with mild to moderate distal proctosigmoiditis (less than 20 cm). The study end point was the determination of clinical, endoscopic, and histologic remission rates at 4 weeks. Eleven patients, nine receiving placebo and two receiving 1.5 g mesalazine, withdrew during trial, mostly because of worsening of symptoms. At 4 weeks clinical remission was achieved in 7 of 31 (39%) patients with placebo, in 22 of 32 (69%) patients in the 1 g mesalazine group, and 23 of 31 (74%) patients in the 1.5 g mesalazine group. No serious clinical or biochemical side effect of treatment was reported. Mesalazine suppositories are safe, well tolerated, and very effective in patients with active distal proctosigmoiditis: 500 mg twice daily appears a suitable dose regimen.