Langerhans cell histiocytosis (LCH) is a clonal proliferative multisystem disease. Although bone and mucosae have been classified as non-risk organs, their involvement may increase the risk of disease progression. Oral and periodontal lesions are burdened with a significant impairment of quality of life for associated signs, symptoms and loss of function. Most of information regards paediatric disease; the disease in adults has received limited attention. Subjects and Methods: A total of 31 adult patients affected by immuno-histopathology confirmed LCH have been prospectively examined; attention was paid to the occurrence and characterization of oral lesions. Results: Twelve patients developed oral lesions. Posterior regions of jawbones were always affected; the involvement of anterior regions was not constant. Unifocal oral involvement was significantly associated with multisystemic disease while multifocal lesions were associated with unisystemic disease. Oral disease presented with soft tissue ulcers (50% of cases), gingival bleeding (66.7%), pain (83.4%), periodontal damage (50%), tooth mobility (16.7%), non-healing extraction socket (8.3%); 41.6% of patients complained of negative outcomes on quality of life. Oral lesions were easily handled with local measures. Conclusions: Posterior regions require attention; single oral lesions may be part of multisystemic disease; oral and periodontal lesions may be early signs of disease reactivation. © 2009 John Wiley and Sons A/S.