Suppositories containing 300 mg 5-aminosalicylic acid (1.96 mmol) or 425 mg acetyl-5-aminosalicylic acid (1.96 mmol) were used in 40 patients with idiopathic proctitis to determine the efficacy of acetyl-5-aminosalicylic acid in treating this bowel inflammation. Each patient was treated with 5-aminosalicylic acid or acetyl-5-aminosalicylic acid suppositories twice daily for 4 weeks in a double-blind trial. Four patients were included twice in the trial. The second time they were treated with the alternative regimen. Six patients in the acetyl-5-aminosalicylic acid group did not complete the trial, four of them because of diarrhoea. Complete clinical remission with normal rectal mucosa on sigmoidoscopy was achieved in 10 out of 18 patients on 5-aminosalicylic acid and in only two out of 15 in the acetyl-5-aminosalicylic acid group (P = 0.03). A favourable histological improvement was demonstrated with 5-aminosalicylic acid suppositories, but the difference with acetyl-5-aminosalicylic acid was not significant (P = 0.059). Three of the four patients who received both drugs recovered with 5-aminosalicylic acid; in none of them was acetyl-5-aminosalicylic acid effective. The results from this study and from previous investigations show that acetyl-5-aminosalicylic acid is not superior to placebo.