A case of pulmonary dirofilariasis in a human being is presented in which the lesion was visible roentgenographically for eighteen months prior to resection and in which the correct etiological diagnosis was made on frozen-section examination at the time of thoracotomy. The documented geographical spread of dirofilariasis in the canine population in the United States and southern Canada suggests that pulmonary dirofilariasis in human beings may be seen with increasing frequency by thoracic surgeons. The epidemiological and pathogenetic features of this unusual disease are discussed. A serological test for dirofilariasis is now available that may allow some patients to escape thoracotomy for this self-limited disorder.