The published data on 60 patients with solitary pulmonary nodules caused or presumed caused by Dirofilaria immitis acquired in the United States are reviewed to characterize the clinical and parasitological features of this disease. Thus far, cases have been reported from 15 states and their geographic distribution has followed that of canine dirofilariasis. Infections have occurred in males twice as often as in females and 56% of all infections have presented in the age group 40-59 years. There has been no apparent racial preference. The presence or absence of symptoms have followed a random distribution and have not been related to the location of the nodule in the lungs, the age, or the sex of the patients. The distribution of the nodules in the lungs also has been random and unrelated to the age or sex of the patients. Fifty-six patients (95%) had a single nodule and three (5%) had two nodules in separate lobes. Ninety percent of the nodules contained a single worm but occasionally two or three worms have been present in the same nodule. The transverse diameter of the worms has ranged between 100 and 400 microns. No single laboratory procedure has been useful in the preoperative diagnosis of these nodules. By radiographic measurement 76% of the nodules were 2 cm or less in diameter. Only seven patients (11.6%) were potentially immuno-compromised. Serial observations in four patients suggest that the lesions are radiographically stable and may eventually become calcified.