Purpose : The purposes of this study are to discribe the high-resolution computed tomographic (HRCT)findings of pulmonary histiocytosis X and to evaluate the relationship between the extent of the lesion on HRCT and pulmonary function test (PFT).Materials and Methods: We retrospectively analyzed the HRCT findings of 8 patients with pathologically proved pulmonary histiocytosis X. The morphological pattern and incidence of the lesions were evaluated. The extent of the lesions were correlated with the results of PFT by calculating Spearman correlation.Results : The most common HRCT findings were thick-walled cysts (n=8) followed by nodules (n=7). Other associated findings were reticulation, ground-glass opacification, and conglomerate nodules. The extent of the disease on HRCT was correlated inversely with percent predicted value of function al vital capacity.Conclusion: Common HRCT findings of pulmonary histiocytosis X were cysts with irregular wall thickening and nodules. The extent of the disease on HRCT correlated with the severity of restrictive pattern of PFT.