Langerhans' histiocytosis or histiocytosis X is a rare intrinsically benign disease producing a destructive tumor with a variable clinical presentation and an often unpredictable clinical course. Focal forms such as eosinophilic granuloma of the bone only require minimal care but the gravity of multisystem forms causing organic dysfunction sometimes require aggressive chemotherapy. Bone involvement is generally observed in children mostly boys. Both sporadic and chronic forms are noted. We report a case observed in a 17-year-old adolescent who presented an exceptional association of bony destruction of the pelvis with extended asymptomatic pulmonary involvement. The lung disease led to the initial diagnosis and optimal surgical, pathological and radiological management.