Retrograde colonic distribution of a new 4 g 5-aminosalicylic acid enema (mesalazine) was investigated in seven patients with ulcerative colitis, five of whom were in remission. The enema was labeled with 99m-technetium and imaged by a gamma camera. A median of 86 percent (range 57-90) of the enema had spread beyond the rectum during the first two hours after administration. In four of the five examined patients with left-sided ulcerative colitis, the diseased mucosa was covered within the first 4 hours. In the fifth patient, with involvement of the descending colon, the enema did not spread beyond the very tortuous sigmoid colon 4 or even 8 hours after enema administration.