Mesalazine is often used as first-line therapy for ulcerative colitis. Several reports have pointed to systemic adverse reactions associated with this drug. Most have evoked a drug-induced hypersensitivity syndrome, while some have described lupus syndromes but with limited clinical and varied biological features. A 75-year-old man presented with fever, dyspnoea, chest pain, polyarthralgia, and myalgia, following mesalazine introduction. Clinical symptoms and low-titre positive antihistone antibodies disappeared after mesalazine withdrawal without recourse to steroids. Pericardial effusion and 8F-fluorodeoxyglucose uptake on positron emission tomography/CT scan, and glomerular haematuria and proteinuria also disappeared. Cytokine-lymphocyte transformation tests showed a strong sensitisation pattern with interleukin-5 production. This case advances our knowledge of the mechanism of mesalazine-induced adverse effects, namely via drug-induced hypersensitivity with lupus manifestations, which we are the first to report.