Shock due to a hypersensitivity response to azathioprine is unpredictable, occurs seldom and bears a potentially fatal outcome. Azathioprine is widely used in the treatment of autoimmune diseases and in solid organ transplantation. Here, we present a patient who suffered from undifferentiated erosive oligoarthritis and was treated with azathioprine. This patient developed anaphylactic shock which was interpreted as a side effect of azathioprine. Although rare, similar cases were described since 1980.