LCH is more common in children but it is often seen in adults too. The disease will be classed as either single-system or multi-system depending on how many of the body’s systems are effected. In a study of 314 people treated for LCH at the Mayo Clinic (Howarth et al, Cancer 1999;85:2278-90) 69% had single-system disease and 31% had multi-system LCH. Approximately half of those treated at Mayo were under 25 years old at diagnosis. Of people with single-system disease the system involved was bone (52%), pulmonary (lung) (40%), skin/mucous membrane (7%) and other sites (1%). Bone involvement was more common in younger patients while pulmonary involvement was mostly seen in those over 15 years old at diagnosis.