Erythema multiforme (EM) in children is understudied and confused with Stevens-Johnson syndrome (SJS) despite their being separate diseases with unique aetiologies and clinical presentations. The goal of this study was to determine the prevalence of Mycoplasma pneumoniae in paediatric patients with EM minor, EM major (EMM), and SJS. This retrospective cohort at The Hospital for Sick Children accrued all cases of EM minor, EMM, and SJS from 1999 to 2013. Sixty-five cases were identified: 20 of EM minor, 23 of EMM, and 22 of SJS. Aetiologies were attributed in 58% of cases: 79% infection and 21% drug aetiology. Sixty-one percent of patients with EMM were M pneumoniae positive, compared with 14% of those with SJS and 22% of those with EM minor (P < .01). M pneumoniae patients were older at presentation (P = .03) and more frequently had sore throat (P < .01) and atypical targets with central blistering (P < .01). These findings suggest that M pneumoniae should be suspected and treated until laboratory confirmation becomes available in patients presenting with atypical target lesions with central blistering.