Pulmonary actinomycosis--a master of disguise.

Three cases of pulmonary actinomycosis are described. In all three cases, an initial diagnosis of pulmonary malignant neoplasia was made. The correct diagnosis was made histologically after the three patients underwent either lobectomy or pneumonectomy. All patients recovered after appropriate therapy. Pulmonary actinomycosis should always be considered in the differential diagnosis of pulmonary neoplasia, especially in young adults.