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Adverse reactions to mesalamine, a treatment used to induce and maintain remission in inflammatory bowel diseases, particularly ulcerative colitis, have been described in the literature as case reports. This case illustrates an unusual adverse reaction. Our patient developed an isolated fever of unexplained etiology, which was found to be related to mesalamine treatment. A 22-year-old patient diagnosed with ulcerative colitis developed a fever with rigors and anorexia 10 d after starting oral mesalamine while his colitis was clinically resolving. Testing revealed no infection. A mesalamine-induced fever was considered, and treatment was stopped, which led to spontaneous resolution of the fever. The diagnosis was confirmed by reintroducing the mesalamine. One year later, this side effect was noticed again in the same patient after he was administered topical mesalamine. This reaction to mesalamine seems to be idiosyncratic, and the mechanism that induces fever remains unclear. Fever encountered in the course of a mesalamine treatment in ulcerative colitis must be considered a mesalamine-induced fever when it cannot be explained by the disease activity, an associated extraintestinal manifestation, or an infectious etiology.