Pulmonary Langerhans cell histiocytosis (LCH) manifests with dyspnoea and a cough with no significant expectoration, with spontaneous pneumothorax being the first symptom in some patients. The disease is caused by multiple granulomas in terminal bronchioles, visible on high resolution CT (HRCT) as nodules. During the further course of the disease, these nodules progress through cavitating nodules into thick-walled and, subsequently, thin-walled cysts. LCH may affect the lungs only or multiple organs simultaneously. Pulmonary LCH may continually progress or remit spontaneously. Treatment is indicated in patients in whom pulmonary involvement is associated with multi-system involvement or when a progression of the pulmonary lesions has been confirmed. To document the disease progression, examination of the lungs using HRCT is routinely applied. Increasing number of nodules suggests disease progression. However, determining the number of nodules is extremely difficult. Measuring radioactivity of the individual small pulmonary loci (nodules) using PET is not possible due to the high number and small size of the nodules. Our centre has a register of 23 patients with LCH; the pulmonary form had been diagnosed in 7 patients. A total of 19 PET and PET-CT examinations were performed in 6 of these patients. PET-CT was performed using the technique of maximum fluorodeoxyglucose accumulation in a defined volume of the right lung--SUV(max) Pulmo. In order to compare the results of examinations performed using the same and different machines over time as well as in order to evaluate pulmonary activity, the maximum fluorodeoxyglucose accumulation in a defined volume of the right lung (SUV(max) Pulmo) to maximum fluorodeoxyglucose accumulation in a defined volume of the liver tissue (SUV(max) Hepar) ratio (index) was used. The disease progression was evaluated using the SUV(max) Pulmo/SUV(max) Hepar index in the six patients with pulmonary LCH. The index value was compared to other parameters characterising the disease activity (HRCT of the lungs, examination of pulmonary function and clinical picture). The SUV(max) Pulmo/SUV(max) Hepar index correlated closely with other disease activity parameters. The traditional PET-CT examination is useful in detecting the LCH loci in the bone, nodes and other tissue but not in the presence of diffuse involvement of pulmonary parenchyma. Measuring the maximum fluorodeoxyglucose accumulation in a defined volume of the right lung and expressing this activity as the SUV(max) Pulmo/SUV(max) Hepar index appears to be a promising approach. Our initial experience suggests that the results obtained using this method correlate well with other parameters that characterise activity of pulmonary LCH. However, this is a pilot study and further verification is required.