A 50 year old male smoker was admitted with nonproductive cough, weight loss and bilateral infiltrates predominantly in the upper lung zones on chest X-ray. Before admission he had been treated for suspected tuberculosis without improvement. The definite diagnosis of pulmonary histiocytosis X was made by open lung biopsy. High resolution computed tomography revealed the typical "ring fixtures" distributed in the parenchyma. Diffusing capacity for carbon monoxide was markedly reduced, besides mild obstruction. After six months of observation, there was seen a radiological improvement with unchanged lung function parameters.