Stevens–Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) are severe cutaneous adverse drug reactions. Recent studies have revealed that the prevalence of SJS/TEN is associated with genetic backgrounds, such as polymorphisms in human leukocyte antigens (HLAs). However, non-genetic factors contributing to the etiology of SJS/TEN are largely unknown. This study aimed to assess the involvement of concurrent infection on the pathological states of SJS/TEN, examining the severity of cutaneous symptoms and ocular involvement as well as the time to onset in drug-induced SJS/TEN patients. We recruited 257 Japanese SJS/TEN patients from June 2006 to September 2013 through a nationwide case collection network and participating hospitals and reviewed the clinical information including patient backgrounds, primary disease and medication status. Association between infection and pathological states of SJS/TEN was assessed using univariate and multivariate analyses. The concurrent infectious group of SJS/TEN patients showed a significantly higher rate of exhibiting severer dermatological and ophthalmological phenotypes and an earlier onset of SJS/TEN than the non-infectious group. Our results suggest that the infection could be a risk factor to cause severer symptoms and earlier onset of SJS/TEN.