A 60-year-old man was admitted to our hospital complaining of general malaise. Examination of arterial blood gases on room air revealed hypoxia. Pulmonary function test showed restrictive abnormality. Chest high-resolution CT showed diffuse mosaic attenuation without evident pulmonary artery abnormality on contrast chest CT. Based on these findings, interstitial pneumonia or chronic pulmonary thromboembolism was suspected. The findings of bronchoalveolar lavage revealed 4.4×10⁵ cells/mL, including 89.6% macrophages, 9.4% lymphocytes, and 1.0% neutrophils. TBLB showed marked alveolitis. Moreover video-assisted thoracoscopic surgical biopsy was performed. Biopsies of the lung specimen showed focal infarct with surrounding mild mononuclear cell infiltrates (homogenous cellular alveolitis). ^99mTc pulmonary perfusion and ^81mKr ventilation scintigraphy showed V/Q mismatch. Furthermore, pulmonary angiography also revealed inadequate artery flow corresponding to the mismatch area of scintigraphy. Collagen vascular diseases and abnormality of coagulation factors were not detected. Multiple perfusion defects persisted for more than 6 months. Thus, finally the patient was diagnosed with chronic pulmonary thromboembolism, pathologically showing homogenous cellular alveolitis.