Aspiration biopsy is a widely accepted diagnostic modality for intrathoracic lesions. Between August 1978 and July 1980, percutaneous needle aspiration biopsy was utilized as a means to diagnose intrathoracic lesions in 82 patients. Cytologic diagnosis was obtained in 81 of 82 patients, 60 with a diagnosis of malignant lesions and 21 of nonmalignant lesions. Forty-four patients (53.7%) were treated surgically and 38 patients (46.3%) were treated conservatively, including radiotherapy and/or chemotherapy. Of the 44 patients who came to surgery, 35 had a malignant cytologic diagnosis. Surgery was performed in 11 patients with an initial benign cytologic diagnosis--there were 4 benign tumors, 4 inflammatory lesions and 3 false-negative cases. Of the 3 negatives, two patients had been biopsied on a second occasion with a malignant cytologic diagnosis. Of the 38 patients treated conservatively (malignant 27, benign 10, insufficient specimen 1) only one patient initially considered benign was later diagnosed with unresectable malignancy. The full collaboration of the radiologist and cytologist at the time of the biopsy has contributed to 94% sensitivity, 100% specificity, and 100% predictive valve for aspiration biopsy of intrathoracic lesions.