Pulmonary dirofilariasis (PD), caused by Dirofilaria immitis (D. immitis), the dog heartworm, is not common in humans, though we recently encountered 4 cases. Chest X-ray images from annual health examinations showed a single spherical nodule in the inferior or middle portion of the right lung in each patient. None of the patients showed any clinical symptoms and had no contact with dogs. Hematological results in 3 of the cases were within normal limits, while mild eosinophilia was found in one. Serological tests for the Anti-Dirofilaria antibodies were not performed. There were no characteristic clinical manifestation of PD in any of the patients, however, we consider it important to keep a diagnosis of PD in mind, when we experienced these cases, they present no characteristic clinical manifestations. Pathologically, macroscopic findings showed well-circumscribed nodules that were round peripheral lesions in lungs. Histological results revealed coagulation necrosis with fibrosis and granulation in the nodule edge, which contained inflammatory cells. By means of silver staining, the worm structures in the nodules could be identified well, and the quadrant cells in the sections were numbered about 30. Immunohistochemically, the somatic muscle tissues were stained with anti-Dirofilaria antibody. These findings indicated that the pulmonary lesions in all 4 cases were due to D. immitis.