Summary Background High‐mobility group box 1 ( HMGB 1) is a damage‐associated molecular‐pattern protein. Stevens–Johnson syndrome ( SJS )/toxic epidermal necrolysis ( TEN ) are serious, immune‐mediated skin‐blistering conditions. Objectives To determine serum and/or blister‐fluid total HMGB 1 levels in SJS / TEN cohorts, and HMGB 1 expression in formalin‐fixed, paraffin‐embedded ( FFPE ) SJS / TEN skin vs. healthy and maculopapular exanthema ( MPE ) skin. Methods Serum HMGB 1 was quantified in Malawian nevirapine‐induced hypersensitivity, Taiwanese SJS / TEN and Spanish SJS / TEN cohorts. FFPE skin (healthy skin, MPE , SJS / TEN ) was stained and assessed for HMGB 1 expression. Results Serum total HMGB 1 was not significantly elevated in patients with nevirapine‐induced SJS / TEN (3·98 ± 2·17 ng mL −1 ), MPE (3·92 ± 2·75 ng mL −1 ) or drug reaction with eosinophilia and systemic symptoms (4·73 ± 3·00 ng mL −1 ) vs. tolerant controls (2·97 ± 3·00 ng mL −1 ). HMGB 1 was significantly elevated in Taiwanese patients with SJS / TEN , highest during the acute phase (32·6 ± 26·6 ng mL −1 ) vs. the maximal (19·7 ± 23·2 ng mL −1 ; P = 0·007) and recovery (24·6 ± 25·3 ng mL −1 ; P = 0·027) phases. In blister fluid from Spanish patients with SJS / TEN , HMGB 1 (486·8 ± 687·9 ng mL −1 ) was significantly higher than in serum (8·8 ± 7·6 ng mL −1 ; P <0·001). Preblistered SJS / TEN skin showed decreased epidermal nuclear HMGB 1 expression in upper epidermis vs. healthy or MPE skin but retained basal/suprabasal expression. ...