Over a period of 38 months, diagnostic needle aspiration of 400 (293 malignant and 107 benign) pulmonary lesions was performed with 20-gauge needles. Deep as well as peripheral nodules and masses were routinely biopsied. Positive diagnostic accuracy was 96.5%. There were 9 (2%) incorrect diagnoses with 5 false negatives and 4 false positives. In almost all patients with malignant pulmonary neoplasms, the specific cell type was identifiable. Pneumothorax was a frequent complication, but there were no fatalities, and no episodes of major bleeding or hemoptysis. A new slotted 20-gauge thin wall needle was used in 258 patients which enabled aspiration of larger amounts of material than had been possible with the standard 20-gauge thin wall needle, and in approximately 50% of patients, enough material was obtained to perform tissue sections as well as smears for cytology. In 77 patients with benign lesions and in 114 with unresectable neoplasms, needle aspiration established the diagnosis and made it unnecessary to perform surgery and/or mediastinoscopy.