In this retrospective review of 541 patients with Langerhans' cell histiocytosis, 211 (39%) patients were older than 21 years of age, whereas 330 (61%) were younger than 21 years of age. The adult patients had a mean age of 32 years (range, 21-69 years) with 159 (75%) men and 52 (25%) women, whereas the pediatric patients consisted of 176 (55%) boys and 144 (45%) girls. This male predominance in adults was statistically significant. Three adults had the Hand-Schuller-Christian variant, whereas the remaining adults (208) had eosinophilic granuloma. The rib accounted for 25% of the adult lesions and only 8% of the pediatric lesions. Spine involvement was less common in the adult group (3% versus 10%) and was predominantly thoracic. The adult patients had 40 (77%) diaphyseal lesions, 12 (23%) metaphyseal lesions, and no epiphyseal lesions. The pediatric patients had 75 (54%) diaphyseal, 59 (42%) metaphyseal, and five (4%) epiphyseal lesions. Radiographic evaluation revealed similar margin and matrix patterns in both groups, with a geographic lesion without sclerotic borders being the most common pattern. Langerhans' cell histiocytosis is considered a pediatric disease. However, this study showed a significant number (39%) of patients older than 21 years of age with this condition.