Objective: Langerhans cell histiocytosis (LCH) has been well described only in children. We analyzed the characteristics, reactivation, and outcome of LCH in a cohort of 55 patients across all ages. Methods: We reviewed the records of all patients with LCH treated at a single institute between Jan. 1974 and May 1998. Results: The 55 patients were 2 to 67 years of age (median, 31 years) at the time of diagnosis, and 85.5% were male. Forty patients (72.7%) had single-system LCH; Fifteen (27.3%) had multisystem disease. The head and neck was the most frequent tumor site (63.6%). LCH was not found in organs at risk of involvement (liver, spleen, bone marrow, and lungs). The frequency of bony invasion (23.6% overall) differed significantly according to age (15 years (66.7%) vs. Age >15 years (11.6%) (P=0.0005). At a median follow-up of 12 years, no patient died of LCH. The 5, 10-year survival estimates were 100%. The 5, 10-year disease-free survival estimates were 70.9% and 58.4%. The 5-year disease-free survival estimate was 58.3% for age ( 15 years vs. 74.4% for age >15 years (P=0.83) and 75% for single-system disease vs. 60% for multisystem disease (P=0.13). LCH was reactivated in 43.6% of patients, with a median of 14 months (range, 2-180 months). Three patients with recurrent disease experienced spontaneous remission. At the time of the most recent follow-up, 23.6% of survivors had active disease. Conclusion: LCH is not found exclusively in children and adolescents. The frequency of bone invasion is inversely related to age. Reactivation is very common regardless of the type of treatment, but the prognosis is generally good.