We evaluated the efficacy of the lateral decubitus position in preventing the development of pneumothorax after percutaneous needle biopsy of a well defined lung lesion. Fifty patients had such a procedure and were immediately placed in the lateral decubitus position for ten minutes, with the biopsied lung below. A chest x-ray film was obtained after the ten minute period, four to six hours later, and on the following morning. The number of pneumothoraces and the type of treatment needed in each case were recorded. Results in these 50 patients were compared to those previously obtained in 132 patients who had had an identical procedure and follow-up during the preceding four years. In all patients included as controls, we did the biopsy using an identical technique and in accordance with a specific written protocol. Both groups of patients were closely comparable in age and sex distribution, baseline values for arterial blood gases, results of prebiopsy tests of pulmonary function, and type, size, location, and distance from the pleural surface of the pulmonary lesions. A postbiopsy pneumothorax developed in 21 of 50 (42%) study patients, and in 46 of 132 (35%) controls (P not significant). Specific therapy was needed in ten (48%) and 26 (56%) of these cases, respectively (P = NS). We conclude that placing patients in the lateral decubitus position is not effective in significantly reducing the incidence of postbiopsy pneumothorax or the proportion of patients who will require specific therapy for this complication.