The present invention provides a method of predicting the risk of a patient for developing adverse drug reactions, particularly SJS or TEN. It was discovered that an HLA-B allele, HLA-B* 1502, is associated with SJS/TEN that is induced by a variety of drugs. The correlation with HLA-B* 1502 is most significant for carbamazepine-induced SJS/TEN, wherein all the patients tested have the HLA-B* 1502 allele. In addition, another HLA-B allele, HLA-B*5801, is particularly associated with SJS/TEN induced by allopurinol. Milder cutaneous reactions, such as maculopapular rash, erythema multiforme (EM), urticaria, and fixed drug eruption, are particularly associated with a third allele, HLA-B *4601. For any of the alleles, genetic markers (e.g., HLA markers, microsatellite, or single nucleotide polymorphism markers) located between DRB1 and HLA-A region of the specific HLA-B haplotype can also be used for the test.