[Pulmonary dirofilariasis with cavity formation and spontaneous regression].

A 70-year-old woman was admitted to our hospital for evaluation of a nodular shadow and an apparent cavity in the right middle lung field. Transbronchial biopsy and percutaneous needle biopsy had failed to result in a diagnosis. Serial chest X-ray films revealed slight regression of the nodule without therapy. Pulmonary dirofilariasis was diagnosed after open-lung biopsy and Ouchterlony's double diffusion test. Cavity formation and spontaneous regression are rare in pulmonary dirofilariasis.