Mesalazine therapy for ulcerative colitis has been reported to be effective and safe. Rare cases of mesalazine-induced renal, pancreatic, myo-pericardial, pleuro-pulmonary and haematological toxicity have been described separately. We report a case characterized by the simultaneous presence of fever, pericarditis, peripheral eosinophilia, eosinophilic pneumonia, anaemia and haematuria (together with proteinuria and leukocyturia) due to mesalazine treatment in a patient with ulcerative colitis. No clinical response had been obtained with corticosteroids and various antibacterial agents. When mesalazine treatment was suspended, all symptoms rapidly and totally disappeared, confirming the direct responsibility of this drug in causing these adverse events. We conclude that mesalazine can induce multi-organ hypersensitivity, which must always be considered as a possible adverse effect during treatment with this drug. To resolve this adverse event it is essential to discontinue mesalazine treatment.