Many patients with ulcerative colitis (UC) respond to mesalamine therapy within 8 weeks. Those not achieving remission after 8 weeks are often treated with steroids or other immunosuppressive therapies. This study aimed to determine the effect of 8 weeks' high-dose MMX mesalamine extension therapy in patients with active, mild-to-moderate UC who had previously failed to achieve complete remission in 2 phase III, double-blind, placebo-controlled studies of MMX mesalamine (SPD476-301 and -302). Patients with active, mild-to-moderate UC who did not achieve clinical and endoscopic remission after or=1 point reduction from baseline in sigmoidoscopy score. Overall, 304 patients who entered this acute extension study were evaluated; 59.5% achieved remission at week 8. Remission rates were similar irrespective of prior treatment in the initial acute phase III studies. Most patients with mild-to-moderate UC who fail to achieve remission with up to 8 weeks' initial mesalamine therapy can achieve clinical and endoscopic remission following a further 8 weeks' treatment with high-dose MMX mesalamine therapy, thereby avoiding step-up therapy.