The results of a clinical trial comparing slow-release 5-aminosalicylic acid tablets (Pentasa) and enteric-coated sulfasalazine tablets (Salazopyrin) with regard to the efficacy of maintaining ulcerative colitis patients in remission for 12 mo and with regard to safety of treatment are reported. Seventy-five patients with ulcerative colitis in remission for between 1 mo and 5 yr were included for analysis. Forty-nine men and 26 women, aged between 18 and 79 yr, received either Pentasa t.i.d. (1500 mg) plus Salazopyrin placebo or Salazopyrin t.i.d. (3 g) plus Pentasa placebo daily. Patients were assessed clinically, endoscopically, and histologically before and 3, 6, 9, and 12 mo after the start of treatment. Life-table analysis showed ongoing remission after 6 and 12 mo for Pentasa to be 63% (26 of 41) and 54% (22 of 41) and for Salazopyrin 72% (22 of 31) and 46% (14 of 31). These differences were not statistically significant. Three patients treated with Salazopyrin were withdrawn because of severe erythrodermia, anxiety and backache, and pregnancy, respectively. One patient on Salazopyrin experienced transient rises in serum urea, creatinine, and lactic dehydrogenase and another patient in this group reported slight reversible loss of hair. In the Pentasa group no side effects were recorded. We conclude that Pentasa is a well-tolerated drug, equally effective as Salazopyrin in maintenance of remission of ulcerative colitis.