BACKGROUND: The efficacy of mesalazine enemas depends on intraluminal concentration of the drug and is therefore limited by the enema distribution in the colon. Active ulcerative colitis changes colon motility and this leads to uncertainty about enema spread. AIM: To assess the influence of disease activity on enema distribution, we conducted a physician-blinded, longitudinal study of the retrograde spread of three mesalazine enemas. METHODS: Thirty-one patients with mild to moderate ulcerative colitis were subdivided into three groups, and treated with 2 g mesalazine in 30 mL (group I, n = 10), 4 g mesalazine in 60 mL (group II, n = 12) or 1 g mesalazine in 100 mL (group III, n = 9). All patients received oral mesalazine 500 mg t.d.s. Enemas were labelled by adding 10 MBq (99mTc)technetium-sulphur colloid. Anterior scintigraphic images were taken at the start of the study and after 12 weeks of therapy; retrograde spread was assessed by calculating the percentage of the enema in each colonic segment. RESULTS: The activity score of ulcerative colitis diminished significantly after 12 weeks of treatment, but five patients dropped out of the study. At the start of treatment enema activity in group I was mainly concentrated in the sigmoid (99%); in group II activity was found in the rectum (9%), the sigmoid (61%) and the descending colon (15%); in group III activity was distributed between the sigmoid (66%) and descending colon (25%). The colonic distribution of mesalazine enemas was not influenced by disease activity. CONCLUSION: Volume, but not disease activity, is the important determinant of retrograde colonic spread of mesalazine enemas in ulcerative colitis.