Thoracic actinomycosis is a rare, chronic debilitating disease and it is difficult to diagnose and treat. Material and Method: Between March 1990 to December 2003, 17 patients were diagnosed and treated for actinomycosis in our center. Except 4 patients (involving cervicofacial, and abdominopelvic area), we reviewed the clinical characteristics, diagnosis method, and treatment in 13 patients. Result: In 8 patients, the operation was required for diagnosis and treatment. 7 of 8 patients had tumor-like lesions in radiological findings and they were not distinguished as lung cancer, and 1 of 8 patients was non-responsive to the antibiotics therapy. Among the 5 medically treated patients, 4 of 5 patients were diagnosed by bronchoscopic biopsy and one by CT-guided biopsy. All of them was well-responsive by the antibiotic treatment. Conclusion: Thoracic actinomycosis is a chronic inflammatory disease that respond well to antibiotics (penicillin therapy). It should be diagnosed by the repeated biopsy (CT-guided or bronchoscopic) before starting treatment. However, if the lesion is not distinguished as the lung cancer, non-responsive to the medical treatment, and the patients experience the recurrent hemoptysis, we can consider the operation for the diagnosis and treatment.