Histiocytosis X, a reticuloendothelial disorder, includes the three clinical syndromes: Letterer-Siwe and Hand-Schuller-Christian diseases and eosinophilic granuloma of the bone. Twenty-eight cases of adult onset of histiocytosis X involving the head and neck region were found in the literature during the past 20 years; 15 cases were confined to this area, out of which 13 involved jaw bones. Characteristic signs and symptoms of periodontal disease were observed in 22 out of 28 cases (79%) with the lower molar area as a predominant site. A case report of a non-osseous chronic disseminated histiocytosis X with periodontal involvement is presented. The importance of histiocytosis X, as an entity that should be kept in mind as a differential diagnosis of a process where destructive periodontitis is involved, is discussed.