||To investigate the association of HLA-B*5801 and HLA-Cw*0302 alleles in Thai patients who had allopurinol induced SCAR including exfoliative dermatitis. Other risk factors besides genetic were also investigated. A case - control study was used. The study was performed during December 2009 – March 2010 at Siriraj Hospital, Bangkok. The totals of 82 patients were recruited into the study; 34 patients were recruited from patient with adverse drug reactions from allopurinol. Genotyping of HLA-B*5801 and HLA-Cw*0302 alleles were determined. Within 34 patients with ADR from allopurinol, there were 25 SCAR (SJS/TEN/HSS) cases HLA-B*5801 and HLA-Cw*0302 alleles were found in all 25 patients with SCAR. Odds ratio was 282.2. This study demonstrated that HLA-B*5801 allele was also associated to allopurinol induced exfoliative dermatitis. Other significant risk factors for hypersensitivity to allopurinol besides genetics were female gender and the presence of diabetes mellitus along with chronic renal insufficiency.