Activation of ulcerative colitis with mesalamine has rarely been reported. In case 1, a 34-year-old man was treated with oral mesalamine, resulting in an exacerbation of colitis that rapidly improved with glucocorticoids and mesalamine withdrawal. Oral cromolyn sodium and occasional low-dose prednisone therapy has maintained long-term remission. In case 2, a 28-year-old man receiving prednisone treatment developed chest pain and myalgias 1 week after initiation of mesalamine that resolved on mesalamine withdrawal. A lower dose of mesalamine with continued glucocorticoids resulted in clinical improvement, and both drugs were tapered. Mesalamine sensitivity was documented endoscopically and histologically by evaluating mucosal changes after two mesalamine enemas during a 24-hour period. There was dramatic progression from quiescent disease to active colitis in 24 hours. Mesalamine sensitivity must be included in the differential diagnosis of ulcerative colitis exacerbations. Concurrent steroid therapy can suppress systemic side effects, making the diagnosis even more elusive.