The study proposes to assess whether compared to placebo, CP-690,550 is effective, safe, and tolerable maintenance therapy in subjects with Ulcerative Colitis (UC). The study proposes to assess whether compared to placebo, CP-690,550 maintenance therapy more effectively achieves mucosal healing and improves quality of life in subjects with UC.The study proposes to assess CP-690,550 pharmacokinetic exposure during maintenance therapy in subjects over the age of 18 years with UC.