Antithymocyte globulin (ATG) therapy is an established form of treatment for aplastic anaemia and has also been used as prophylaxis against graft rejection of bone marrow and renal allografts. Administration of ATG preparations has been associated with many mild clinical reactions, as have other forms of immunomodulatory therapy. However, serious adverse effects appear to be rare. We report a case of rapidly progressive fibrosing alveolitis and thyrotoxicosis in relation to ATG therapy, highlighting its potential toxicity and emphasising that its administration should be undertaken by experienced physicians in specialised centres.