We reviewed our experience with 2,114 percutaneous aspiration needle biopsies of intrathoracic lesions. Aspiration was performed for cytological diagnosis employing biplane fluoroscopy and a 20 gauge needle, 0.9 mm in outside diameter. A satisfactory specimen was obtained in 88% of biopsies, and the chance of obtaining a correct diagnosis of a malignant lesion was 81.5%. The false positive rate was 2.3%, and the cytologists could always distinguish between primary and secondary neoplasms. A false negative rare of 13.6% (36 patients) resulted in only three delayed thoracotomies and two instances of interval metastases discovered at mediastinoscopy. Cellular specificity in primary tumors was not sufficiently accurate to affect therapy. Pneumothoraces occurred frequently (31.9% of patients) but wee generally small; 10.4% of patients required chest drainage. There were no recorded instances of tumor implantation in needle tracts. We conclude that a rapid and accurate diagnosis of intrathoracic pathology can be obtained by this technique. It is associated with an acceptable morbidity and may greatly expedite both patient care and investigation.