We report here the case of a 51-year-old man presenting to the Emergency Department with a febrile cutaneous eruption with diffuse arthralgia 10 days after the onset of azathioprine therapy. The clinical examination did not reveal any inflammatory syndrome and the results of all bacteriological tests were negative. A skin biopsy was performed, which revealed a granulocytary pustula with superficial dermal oedema and a neutrophil infiltration without sign of vasculitis. A side effect of azathioprine was suspected, and treatment was discontinued. Fortunately, the patient recovered within a few days. Azathioprine hypersensitivity syndrome is a rare side effect of azathioprine. Hypersensitivity syndrome is an idiosyncratic, non-IgE-mediated reaction that appears to be unrelated to thiopurine methyltransferase levels. Diagnosis is mainly clinical and requires an exclusion of other processes. The only treatment option available is to stop azathioprine intake.