The efficacy and safety of 4 g 5-aminosalicylic acid (5-ASA) enemas were assessed in 59 patients with ulcerative colitis involving up to 50 cm of their distal colon. Twenty-nine patients received 5-ASA and 30 received a placebo. There were 12 dropouts (five in the active and seven in the placebo group) during the study because of insufficient efficacy. After six weeks of therapy, 63% of the patients receiving the 5-ASA were considered to be "much improved" by the study physician compared to 20% patients on placebo (P less than 0.0001). A disease activity index (DAI), based upon patient symptoms and sigmoidoscopic appearance, was used to assess efficacy. Mean DAI declined 75% for patients on 5-ASA enemas and 32% for patients on placebo (P less than 0.05). The 5-ASA enemas are well tolerated and are of benefit in the treatment of ulcerative colitis confined to the distal colon.