The efficacy and safety of 4-g 5-aminosalicylic acid enemas were assessed in 153 patients with ulcerative colitis involving up to 50 cm of distal colon. Seventy-six patients received active medication and 77 received a placebo. There were 20 dropouts (6 in the active group and 14 in the placebo group) during the study because of insufficient efficacy. After 6 wk of therapy, 48 of the 76 patients (63%) receiving 5-aminosalicylic acid were considered to be "much improved" by the study physician compared to 22 of the 77 patients (29%) on placebo (p = 0.001). A disease activity index based on patient symptoms and sigmoidoscopic appearance was used to assess efficacy. Mean disease activity index declined 55% for patients on 5-aminosalicylic acid and 24% for patients on placebo (p = 0.0001). Analysis of subgroups indicated that patients most likely to respond were those with disease confined to the 20-40 cm from the anus. Response was not affected by concurrent sulfasalazine, but patients requiring concurrent oral steroids had a diminished response. Rapid onset of efficacy was shown by a significant reduction in rectal bleeding within 3 days of treatment initiation. 5-Aminosalicylic acid enemas are well tolerated and are of benefit in the treatment of ulcerative colitis confined to the distal colon.