Human infection with the dog heartworm (Dirofilaria immitis) may lead to focal pulmonary infarction with granuloma formation. The resulting roentgenographic coin lesion may require a diagnostic thoracotomy in consideration of malignancy. Because of sometimes enigmatic histopathological characteristics, this process may not be receiving the recognition it deserves. Furthermore, the dramatic increase of primary (canine) host infections in the United States presages an increase of secondary (human) host infections. Many thoracotomies will be performed for this innocuous process unless the dirofilarial agent can be controlled or the human pulmonary lesion can be reliably identified without operation.