We report a 14-year-old boy with Stevens-Johnson syndrome who presented with mucous membrane lesions but without skin lesions and discuss the differential diagnosis of oral mucous membrane lesions in childhood. Stevens-Johnson syndrome in children is most frequently caused by a Mycoplasma pneumoniae infection. The full clinical picture of Stevens-Johnson syndrome can be present before seroconversion of Mycoplasma antibodies is observed. One should keep in mind that one negative titer of Mycoplasma antibodies does not rule out M. pneumoniae infection.