Thymus gland involvement in Langerhans cell histiocytosis (LCH) is usually part of multi-system disease and may be more common than previously recognized. However, thymic involvement causing an anterior mediastinal mass is an extremely rare presentation of multisystem LCH. Here we report a 2-month-old-boy admitted to hospital with a giant anterior mediastinal mass with multisystem LCH involving the thymus, lungs, liver and skin. The differential diagnosis of mediastinal mass in children should also include LCH, especially multisystem disease. LCH should also be kept in mind in the differential diagnosis of skin lesions in infants, even if spontaneous regression occurs.