Advances in genetic research and molecular biology techniques have made it possible to begin to characterize the underlying genetic factors that predispose patients to serious forms of drug-induced skin injury (DISI). To facilitate research in this area, we have set out standardized phenotypic definitions for (i) Stevens-Johnson syndrome/toxic epidermal necrolysis (SJS/TEN), (ii) acute generalized exanthematous pustulosis (AGEP), and (iii) hypersensitivity syndrome (HSS; also known as drug reaction with eosinophilia and systemic symptoms (DRESS) and drug-induced hypersensitivity syndrome (DIHS)). A DISI Expert Working Group comprising participants with varied expertise reviewed and debated current terminology and diagnostic criteria for DISI and agreed on the minimum phenotypic criteria for selected forms of DISI (SJS/TEN, AGEP, and HSS). In addition, an algorithm has been developed to aid appropriate clinical categorization of patients with DISI. These standardized criteria will be important in facilitating adequate and accurate patient recruitment in order to advance research in pharmacogenomic, immunological, mechanistic, and epidemiological studies.