 Diagnosis is mostly based on the clinical appearance and the medical history . The most important diagnostic feature is a history of recurrent , self healing ulcers at fairly regular intervals . Although there are many causes of oral ulceration , recurrent oral ulceration has relatively few causes , most commonly aphthous stomatitis , but rarely Behçet 's disease , erythema multiforme , ulceration associated with gastrointestinal disease , and recurrent intra @-@ oral herpes simplex infection . A systemic cause is more likely in adults who suddenly develop recurrent oral ulceration with no prior history . 
