BACKGROUND High-dose (4.0 g/day) mesalazine is typically used for induction therapy, but its efficacy as maintenance therapy remains to be determined. We conducted a multicenter retrospective study to investigate the efficacy of continuous treatment with 4.0 g/day of mesalazine. MATERIAL/METHODS Japanese ulcerative colitis (UC) patients receiving acute induction therapy with 4.0 g/day mesalazine were enrolled and followed. Those who clinically improved or who achieved clinical remission were categorized into 2 sub-groups according to the median duration of treatment with 4.0 g/day of mesalazine. The clinical relapse frequency and the time to relapse were analyzed. RESULTS We enrolled 180 patients with active UC, and then 115 patients who clinically improved or who achieved clinical remission after treatment with 4.0 g/day mesalazine were categorized into 2 sub-groups according to the median of treatment duration: a short-term treatment group (≤105 days, n=58) and a long-term treatment group (>105 days, n=57). Overall, 45 (39.1%) patients relapsed: 28 (48.3%) in the short-term treatment group and 17 (29.8%) in the long-term treatment group. This difference was statistically significant (p<0.05). The relapse-free rate in the long-term treatment group was significantly higher than that in the short-term treatment group (p<0.05). The mean time to relapse in the long-term treatment group was significantly longer than that in the short-term treatment group (425.6±243.8 days vs. 277.4±224.5 days; p<0.05). CONCLUSIONS Long-term continuous treatment with high-dose mesalazine (4.0 g/day) may be more effective than short-term treatment for maintenance of remission in UC patients.