Fifty-six patients with ulcerative colitis of mild to moderate severity were entered into a randomized, double-dummy comparison of sulphasalazine, 3 g/day, with olsalazine, 3 g/day. Patients were assessed clinically, and by sigmoidoscopy and biopsy, on entry and at 5 weeks. Both agents produced a similar reduction in stool frequency and in the passage of blood and mucus. Improvements in sigmoidoscopic and histological appearances of the rectal mucosa were observed to a similar extent in both groups of patients. Two patients treated with olsalazine were withdrawn because of increased diarrhoea attributable to the medication. Two patients given sulphasalazine for the first time developed a skin rash. Other side-effects seen during the trial were mild. In this small short-term study, oral olsalazine appeared to be as effective as sulphasalazine in the treatment of mild to moderate ulcerative colitis.