Percutaneous transthoracic needle aspiration was performed on 228 occasions to obtain lung specimens from 202 patients with suspected Pneumocystis carinii pneumonitis. In 121 patients the diagnosis was established by identifying P carinii organisms in lung aspirates. Six patients whose aspirates did not contain P carinii were found to have the organism at autopsy. Findings from toluidine blue O and Gomori methenamine silver nitrate stains were equally satisfactory for detecting P carinii, but the percentage of specific diagnosis was higher when specimens were stained with both. Pneumothorax that required a thoracotomy tube occurred in 39 patients. Other infectious agents, either bacteria or fungi, were found in only four patients. Percutaneous pulmonary needle aspiration--when performed under fluoroscopic guidance--is a rapid and effective method for the diagnosis of P carinii pneumonitis.