A prospective double blind and randomized study was conducted to compare 4-aminosalicylic acid (4-ASA) and prednisolone-21-phosphate enemas in inducing remission in patients with acute distal ulcerative colitis. Patients with ulcerative colitis distal to the splenic flexure as assessed by flexible colonoscopy, barium enema and histology were included in the study. Of 40 consecutive patients, 20 were randomized to each of the two treatment groups. Clinical evaluation was done weekly; sigmoidoscopy and histology were performed at entry and at the end of 4 weeks. Therapy was discontinued in four patients treated with prednisolone enemas due to worsening of symptoms. The clinical improvement was significant in the remaining patients (P less than 0.001) and was similar in the two groups (P greater than 0.1). Sigmoidoscopic and histological improvement were better with 4-ASA than with prednisolone enemas. No adverse effects were observed in any of the patients treated. The present study suggests that 4-ASA is a safe and effective treatment for inducing remission in acute distal ulcerative colitis.