Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) may be fatal. Although classified by body surface area skin detachment, initial stages of both may present with erythema multiforme (EM)-like lesions. To diagnose and predict disease activity adequately as early as possible for patients revealing EM-like lesions, we performed frozen-section diagnosis. Thirty-five patients clinically diagnosed as EM, SJS or TEN were biopsied to diagnose and predict disease progression within the initial-visit day. Half of a histological section taken from a lesion was snap-frozen and immediately cryostat-sectioned, acetone-fixed and stained with hematoxylin-eosin. Specimens were examined with light microscopy for presence of epidermal necrosis. A section from unaffected sites was also examined for 11 patients. Specimens were examined with light microscopy for presence of graft-versus-host reaction (GVHR)-like findings: apoptotic keratinocytes and satellite cell necrosis. Epidermal necrosis was seen in nine patients. Initial diagnosis of the nine was one of overlap SJS-TEN, four of SJS and four of EM, and final diagnosis of those was one of TEN, one of overlap SJS-TEN, four of SJS and three of EM. Dissociation between initial and final diagnosis was seen in three cases. GVHR-like findings in the epidermis were observed in two patients finally diagnosed as overlap SJS-TEN and TEN. Frozen sections are useful not only to make a diagnosis of erythema multiforme but to assess a potential to exhibit more aggressive clinical behaviors (SJS or TEN).