The purpose of this study is to investigate the relationship between occurrence and inducing factors of herpes labialis developed after periodontal therapy and to suggest prediction model of this lesion. A total of 100 patients were studied. A standard schedule was used for interviews of patients. It included demographic information, patient and familial history of recurrent aphthous ulcer and recurrent herpes labialis, history of systemic disease, religion, and emotional state. In case of female patients, the association of dysmenorrhea and onset of recurrent herpes labialis was also observed. After periodontal therapy, some details about therapy, such as the kind of therapy, location, spending time were recorded. At next appointment, the appearance and location of herpes labialis were examined. The frequency of herpes labialis after periodontal therapy was 8% and the location was predominantly mouth angle. The significant relationship was found between the onset of herpes labialis and the history of recurrent herpes labialis, surgical therapy rather than non-surgical therapy, and spending time. The prediction model of herpes labialis was not apparently established with the results of this study. In conclusion it is suggested that we should minimize traumatic manipulation and treatment time to prevent the onset of herpes labialis after periodontal therapy.