We present the case of a 29-year-old man who came to our clinic with a case of one month's evolution consisting of a daily fever of 39º-40º, intense pleuritic pain in the left hemithorax and cough with abundant blackish expectoration. He had no medical or surgical history of interest. He had been a smoker of 20 cigarettes/day for 10 years. Physical examination was strictly normal, except for pulmonary auscultation in which there was an overall decrease in vesicular murmur with isolated areas of echophonia. Various analytical studies were performed, including: haemogram, general biochemistry, baseline arterial blood gases, creatinine clearance, calciuria, proteinogram, immunoglobulin and complement dosage, C-reactive protein, rheumatoid factor, angiotensin-converting enzyme determination and biological markers, all results being normal. Spirometry showed a restrictive pattern. A pulmonary diffusion test showed decreased diffusion (65%), compatible with a reduction of the exchange surface. The plain chest X-ray showed a bilateral interstitial pattern. A chest CT scan showed multiple cystic lung lesions, none larger than 2 cm, distributed in both lungs, especially in the upper and middle lobes. A lytic lesion was also observed at the level of the left fourth costal arch. The Thoracic Surgery Department was asked to perform an open lung biopsy. Samples were taken from the upper and lower lobes of the left lung. The histopathological study showed infiltration of the lung parenchyma by eosinophils and mononuclear cells with aryronucleated nuclei typical of Langerhans cells. S-100 and CD1a immunohistochemical stains were positive. Electron microscopy was performed and the characteristic Langerhans cells with polylobed nuclei and deep indentations were observed. Structures formed by two double bet membranes were also seen, separated by a dense structure that fulfils the characteristics of a Birbeck granule in its tubular portion. With the diagnosis of LCPH, the patient was advised to stop smoking and an appointment was made for a follow-up chest CT scan at 6 months. In this study, the cystic lesions disappeared almost completely, with the costal lytic image persisting. Clinically, the patient showed a significant improvement and was asymptomatic except for the persistence of a slight costal pain in the left hemithorax that corresponded to the bony lesion observed in the CT scan.