The case report given here describes an unusual case of a 35-year-old otherwise healthy male diagnosed with aggressive form of Langerhans cell histiocytosis initially taking course under the form of lymphoma with expressed B symptoms (night sweats, fever and weight loss) and generalized peripheral lymphadenopathy. Also present were productive cough and perianal itching. The diagnosis was determined from lymph node and perianal skin biopsies. Furthermore, by a typical finding on HRCT (high-resolution computed tomography), pulmonary involvement was confirmed the gradual development of which we succeeded to document through a series of several HRCT and PET/CT scans from its initial florid phase characterized by disseminated nodularities up to the terminal phase with the decline of activity and development of cystic formations. After the collection of peripheral blood stem cells, the planned patient's therapy started which in all consisted of three monotherapy cycles with cladribine followed by three cycles of combined chemotherapy (cladribine + cyclophosphamide + methylprednisolone) and complemented with curative radiotherapy of the perianal area. This treatment put the disease into complete remission. However, in two months the initial B-symptoms occurred again, along with the pulmonary symptomatology, perianal pruritus and newly also hip bone pains. The suspected LCH relapse was proved histologically by lymph node biopsy and confirmed at a restaging PET/CT examination which also showed disease dissemination into the hip bones. Consequently, an aggressive form of the disease with early relapse had been the case, which was indicated for administering 4 cycles of CHOEP (cyclophosphamide + doxorubicin + vincristine + etoposide + prednisone) as salvage regimen completed in March 2010 with autologous peripheral blood stem cell transplantation after high-dose BEAM (carmustine + etoposide + cytarabine + melphalan) chemotherapy. Thus, the generalized involvement of nodes doesn't always need to be malignant lymphoma or metastatic dissemination of a tumour but also LCH may be the case. The presence of B symptoms may very likely stand for an aggressive form of the disease course. Histological evaluation of the proliferative characteristic (given by Ki-67 immunohistochemical proliferative index marker and also morphologically by the number of mitosis) may draw attention to an aggressive form of this disease. However, therapy with cladribine (2-chlorodeoxyadenosine) which proves beneficial in classic forms of LCH, in cases of highly aggressive forms of LCH doesn't need to have the same effect as in LCH with low proliferative activity, which conforms to the present experience in the treatment of indolent and aggressive lymphomas. In our study, the hybrid PET/CT imaging proved high sensitivity in evaluating the activity of the disease, including its early relapse. We are presenting here a new method for description and evaluation of diffuse increased activity of pulmonary parenchyma by means of PET/CT examination and for using this method within the framework of monitoring the curative response.