Objective: Toxic epidermal necrolysis and Stevens-Johnson syndrome have related high morbidity and mortality. We predict that preexisting multimorbidity is a major prognostic factor of both these diseases. Methods: A retrospective analysis in toxic epidermal necrolysis and Stevens-Johnson syndrome patients over the past 10 years. Three severity categories (minor, moderate, and severe multimorbidity) were defined according to a point-rating system. Results: Twenty-seven inpatients, with a median age of 63 years, diagnosed with toxic epidermal necrolysis (n = 13) or Stevens-Johnson syndrome/toxic epidermal necrolysis (n = 14) were assessed in this study. Of these, 14 patients died during the course of the study. Nonsurvivors showed significantly higher multimorbidity (P = .038), with higher scoring on the points system for disease severity (P = .003), than survivors and CART (Classification and Regression Trees) cross-validation (P < .05). Limitations: Restricted number of patients due to low prevalence rate. Conclusion: The complexity of associated multimorbidity appears to have a large influence on toxic epidermal necrolysis and Stevens-Johnson syndrome prognosis, which has not been considered in any of the established scoring systems.