After 5,300 percutaneous transthoracic needle aspiration biopsy procedures in 2,726 patients pneumothorax occurred in 27.2 per cent of the patients. Only 7.7 per cent required exsufflation or drainage. The factors influencing the relative frequency and the severity of pneumothorax are discussed. Bleeding around the punctured lesions was found in 11 per cent, and hemoptysis in 2 per cent but were of no clinical importance. In one case evidence of needle track implantation was found. The theoretical and practical importance of tumour cell spread and spread of infection through the needle track is discussed. No air embolism or mortality occurred.