Transthoracic puncture for peripheral pulmonary lesions not diagnosed by fibroscopy were performed on 70 patients under computed tomographic (CT) control. Sixty four cases had a simple cytological puncture associated in 23 cases with a tru-cut biopsy. Six patients had a needle biopsy. The CT enabled us to localise the tumour to be punctured very accurately, whatever its size or topography in relation to the thoracic wall or mediastinum. It also distinguished necrotic zones within the body of the tumour whose biopsy would lead to false negative results. The sensibility for detecting malign tumours by this method was 83% for cytology and 90% when a biopsy was associated with cytology by needle aspirate. The specificity was 94%. A more precise histological diagnosis was correctly suggested by simple cytology in 7 patients (20%). A tru-cut biopsy enabled histological characterisation in all the neoplastic cases; amongst the benign lesions a specific diagnosis was made on the biopsy in 4 out of 10 cases. There were complications -9% overall. None of the pneumothoraces produced required surgical drainage. There were no fatal complications. In conclusion, transthoracic puncture under CT control is a sensitive method for detecting peripheral malignant tumours and is associated with a low level of complications.