The authors evaluated percutaneous aspiration biopsies of intrathoracic lesions in 348 patients where the obtained cytological results were compared with the histological finding from resections or post-mortem preparations. In 274 patients malignant and in 74 benign disease was confirmed. In the differentiation of malignant and benign disease the sensitivity of cytology was 84.3%, the specificity 91.9% and the accuracy 85.9%. The type of bronchogenic carcinoma was correctly diagnosed by cytological examination in 69.5%, in squamous cell carcinoma in 76.8%, in small-cell carcinoma in 80% and in adenocarcinoma in 59.1%. The same cytological finding as in the primary tumour was recorded in 73.5% of the patients with metastases in the lungs. Less favourable results as regards assessment of the type of tumour were obtained in other tumours, in particular in lymphogranulomas and non-Hodgkin lymphomas. In 25% it proved possible to assess by cytological examination the type of benign lung tumour.