Presence of HLA B*58:01 is associated with hypersensitivity to Allopurinol therapy. Allopurinol is widely used for hyperuricemia related diseases such as Gout, Lesch- Nyhan Syndrome and recurrent urate kidney stones. Such individuals are at risk of developing Allopurinol hypersensitivity syndrome which includes Toxic Epidermal Necrolysis (TEN) or Stevens- Johnson Syndrome (SJS) and also features of eosinophilia, vasculitis and major end organ disease. Patients of Han Chinese or Thai descent are at a very high risk for developing Allopurinol hypersensitivity syndrome. Therapy should be discontinued immediately and permanently if Allopurinol hypersensitivity syndrome develops.