The authors analysed 2,653 cases of transthoracic fine needle aspiration cytology of the lung to evaluate the diagnostic accuracy and its limitation. A comparison was made between the original cytologic and the final histologic diagnoses on 1,149 cases from 1,074 patients. A diagnosis of malignancy was established in 38.3% benign in 48.1%, atypical lesion in 2.3%, and inadequate one in 11.9% of the cases. Statistical data on cytologic diagnoses were as follows; specificity 98.9%: sensitivity of procedure, 76.8%: sensitivity of diagnosis, 95.5%: false positive 5 cases: false negative 18 cases: predictive value for malignancy, 98.8%: predictive value for benign lesion, 79.5%: overall diagnostic efficiency, 87.5%: typing accuracy in malignant tumor, 80%.