Pulmonary function tests and chest radiographs of 160 patients who had had percutaneous needle biopsy of lung lesions were reviewed to determine the value of these examinations in estimating the risk of postbiopsy pneumothorax. Chest radiographs were evaluated subjectively for changes of obstructive and restrictive airway disease and for size and depth of lesion. Pulmonary function tests, consisting of simple spirometry (forced vital capacity, percentage of predicted forced vital capacity, forced expiratory volume in 1 sec, percentage of predicted forced expiratory volume in 1 sec, and [forced expiratory in 1 sec/forced vital capacity] X 100), and the pulmonologist's interpretation were evaluated. Pneumothorax developed in 46% (31/67) of patients who had obstructive airway disease according to the results of pulmonary function tests and in 42% (34/81) of those who had obstructive airway disease according to changes on chest radiographs, compared with 19% (10/53) and 25% (17/67) of those who had normal pulmonary function tests and chest radiographs, respectively. Pneumothorax developed in 46% (23/50) of patients who had findings of obstructive airway disease on both pulmonary function tests and on chest radiographs, compared with 7% (2/28) of patients who were classified as normal by both criteria. None of the patients who had normal pulmonary function tests required placement of a chest tube, whereas 19% (13/67) of those who had obstructive airway disease required chest tubes. Decreasing size of lesion and increasing depth of lesion were associated with a significant increase in the risk of pneumothorax. We conclude that the results of chest radiographs and pulmonary function tests are useful parameters for estimating the risk of postbiopsy pneumothorax.