The article describes the use and results of transthoracic aspiration cytology for diagnosis of thoracic infection in a Thai referral chest center. We reviewed 60 cytologic samples, initially diagnosed as thoracic infection or inflammation among a total of 532 percutaneous transthoracic needle aspirations obtained from patients with clinical suspicion of malignancy in a period of 6 years. Follow-up clinical data were collected and correlated with cytologic diagnosis. We found specific microorganisms in 8 samples (13.33%). These included 4 cases of actinomycosis, 3 cases of cryptococcosis and a case of aspergillosis. Granulomatous inflammation was found in 12 samples (20.00%). Among these patients, 10 cases were verified as having tuberculosis. The remaining 40 samples (66.67%) revealed acute inflammatory exudate with no specific microorganism. Follow-up clinical data confirmed or assumed infection in 27 cases. Therefore, in patients with thoracic infection who presented with clinical suspicion of malignancy, from our experience, aspiration cytology revealed adequate morphology for accurate diagnosis which resulted in prompt specific treatment and better prognosis.