In a sample of 7 patients with pulmonary LCH, we monitored disease activity through repeated PET-CT examinations. The small size of LCH infiltrates in bronchioles (pulmonary nodules) precludes direct measurement of their activity. Therefore, SUVmax (maximum standardized uptake value) in the right lung and SUVmax in the liver were measured, as this was achievable with the available PET-CT software. The maximum activity measured in the right lung to maximum activity measured in the liver ratio was used to express the pulmonary activity of the Langerhans cell histiocytosis, i.e. the SUVmax pulmo/ SUVmax liver index. In total, 25 measurements were performed in 7 patients with pulmonary LCH. This index values correlated with the development of the disease activity evaluated through overall patient status, HRCT imaging of the lungs and according to functional examination of the lungs. So far, this is a pilot project and the usefulness of this index in evaluating activity of the pulmonary LCH will require further verification.