Pulmonary actinomycosis. Case report. Pulmonary actinomycosis is a rare infectious disease. The major difficulty is mainly the diagnosis. A high dose of suspicion is required in each intrathoracic process showing malignant behaviour, despite benign histology. Especially if the process extends through normal tissue planes, actinomycosis should be suspected. The therapy of choice should consist in high dose of antibiotics for several months, occasionally followed by surgery. Often though, surgery is the first treatment because of the unability to differentiate the lesion from malignant tumor. Surgery should always be followed by long term antibiotic therapy.