Langerhans' cell histiocytosis (LCH) of bone is a disorder of histiocytic proliferation with variable and often unpredictable behavior. The authors evaluated the clinical and pathologic features of 263 patients (172 children, 91 adults) with biopsy-proven LCH examined during an 80-year period at the Mayo Clinic. Only patients with bone involvement pathologically and/or radiographically were included in the study. Clinical follow-up was available for 245 patients and ranged from 3 months to 50 years (mean, 12 years; median, 10 years). Chi-square tests were used to determine associations between age, gender, extent of osseous involvement, visceral disease, and pathologic features. Survival analyses were performed by univariate and multivariate Cox regression methods. Age at presentation ranged from 2 months to 71 years with a clear predominance in children. The most common presenting complaint was pain, often worse at night. The skull was the most frequent osseous site in children and adults. Diabetes insipidus was documented in 40 patients. Forty-four children developed skeletal recurrence and/or new bone lesions, 19 of whom had diabetes insipidus. Fourteen children and 3 adults died either directly or indirectly from LCH. One adult patient developed systemic amyloidosis. All but two of these pediatric patients were 3 years of age or younger at presentation. All children with hepatosplenomegaly (7 patients) and/or thrombocytopenia (9 patients) died. Nine of the 14 children who died presented initially with three or more bone lesions. The clinical behavior of LCH of bone is often unpredictable; however, young age at diagnosis, hepatosplenomegaly, thrombocytopenia, and polyostotic (> or = 3 bones involved) disease are associated with a poor prognosis (P < 0.005). Recrudescence in children, but not in adults, strongly correlates with the presence of diabetes insipidus (P < 0.0005).