PURPOSE: Percutaneous fine needle aspiration biopsy is known to be a useful diagnostic method for the diagnosis of various pulmonary diseases. Its diagnostic yield is high, and it is safe, but complications such as pneumothorax can occasionally occur. We reviewed the complications arising after needle aspiration biopsy and analyzed the risk factors of pneumothorax. MATERIALS AND METHODS: The medical records and radiographic studies of 157 patients with various pulmonary diseases who underwent needle aspiration biopsy of the lung between 1990 and 1996 were retrospectively reviewed. The clinical features, treatment, and courses of complications were reviewed, and risk factors of pneumothorax such as depth and size of lesion, diameter of needle, number of punctures, and obstructive pulmonary abnormalities were analyzed. RESULTS: Complications occurred in 40 of 157 cases (25.5%), namely pneumothorax in 26 (16.6%), hemoptysis in 11 (7%), hemothorax in two (1.3%), and recurrence of malignancy at the site of aspiration in one (0.6%). When the patients were divided into three groups according to depth of lesion, there were significant difference in the incidence of pneumothorax ; the results were as follows : less than 2cm, 12.9% ; between 2 and 4cm, 24.1% ; and larger than 4cm, 57.1% (p<0.05). In pulmonary function testing, FVC (forced vital capacity) of patients with pneumothorax was less than that of patients without (2.6±0.9L vs 3.1±0.8L, p<0.05), but FEV1 (forced expiratory volume in 1 second), FEV 1% (percentage of predicted FEV1), FEV1/FVC,and FVC%(percentage of predicted FVC) were not different between the two groups. The incidence of pneumothorax in patients with pleura-attached lesion (9%) was lower than that of those with non-attached lesion (26%, p=0.01). The age of patients, size of lesion, diameter of the needle, guidance methods and number of aspirations showed no significant relationship with pneumothorax. CONCLUSION: In needle aspiration biopsy of the lung, depth of lesion and passage of a needle through aerated lung are significant risk factors of pneumothorax.