We have performed percutaneous needle aspiration biopsy of intrathoracic lesions under fluoroscopic control on 50 patients. Lesions as small as 1 to 1·5 cm. in diameter were aspirated. Thirty of the 36 cases of malignant neoplasm were diagnosed by this method. Of the 14 non-neoplastic cases, granuloma was diagnosed in two, and in eight of the remaining 12 the clinical diagnosis was supported by the nature of the aspirated material. Pneumothoraces are to be expected as a complication of the method. Most will be asymptomatic; a small proportion will require treatment. There will also be the occasional haemoptysis. The procedure can be quickly performed and it is well tolerated by the patient. It causes less discomfort than bronchoscopy or scalene lymph node biopsy. The interpretation of the cytological material presents no undue difficulty to the pathologist experienced in conventional pulmonary exfoliative cytology.