PURPOSE: Transthoracic fine needle aspiration biopsy is a widely practiced technique in the investigation of the lung and mediastinal masses because of its safety, reliability and accuracy. We report the diagnostic accuracy of the transthoracic fine needle aspiration biopsy and the frequency of its complications which required treatment. MATERIALS AND METHODS: We analyzed 1000 transthoracic needle aspiration biopsies whcih were performed in 986 patients with 993 chest lesions. Aspiration biopsies were obtained with Westcott needles (20G or 22G) under the fluoroscopic guidance. Final diagnosis was made with operation, bronchoscopy, sputum study, biopsy of other sites and the clinical course of the patient. We analyzed diagnostic yields of aspiration biopsy and the frequency of the complication requiring treatment. RESULTS: The sensitivity was 87.6% in benign diseases and 95.8% in malignant diseases. Pneumothorax was the most frequent complication, which required pig-tail catheter insertion in 36 cases or thoracostomy in six cases. In 36 cases, minimal hemoptysis developed which did not require treatments. CONCLUSION: We obtained relatively high sensitivities of malignant and benign lesions using transthoracic fine needle aspiration biopsy and the frequency of the complications requiring treatment was very low. Transthoracic fine needle aspiration biopsy is a valuable diagnostic method in the thoracic lesions.