Fifty patients, from 5 weeks to 7) years of age, were subjected to transthoracic thin needle biopsy of their mediastinal or hilar masses from May 1975 to May 1976. The material aspirated was satisfactory for cytological examination in 41 patients (82%). The cytological diagnosis could be made in 36 patients (72%) and could be confirmed by histological examination in 12 patients, and by the clinical course of the disease in 16 patients, while 7 patients are lost to follow-up. A false negative diagnosis was probably made in one patient. The complication rate was low, mainly pneumothorax (in eight patients) and hemoptysis (in six patients) of little clinical significance. The main drawback of the method seems to be its inability to provide enough material for reliable diagnosis of benign lesions.