An investigation was carried out on 271 patients who underwent transthoracic fine needle aspiration biopsy ( FNAB ) for suspected pulmonary lesions in a four-year period; 80% of them had a malignant tumor. The histologic control of the cytologic diagnoses made on the transthoracic FNAB was possible in over 50% of the cases. The sensitivity and predictive value for positive results were, respectively, 0.890 and 0.995, whereas the typing accuracy verified in 58 cases on the surgical specimen was 0.76. The comparison of the data obtained from the most recent case material with that of early observations confirmed the high sensitivity of transthoracic FNAB in the detection and characterization of malignant lesions in the lungs. The sensitivity was almost identical for primary tumors, 0.90, and metastatic disease, 0.88. Furthermore, over the years the data showed an improvement in the results due to the experience gained, the combined use of cytology and histology and the application of histochemical methods. More sophisticated methods, such as immunocytochemistry and electron microscopy, were essential to the final diagnosis in only a few cases.