In percutaneous transthoracic needle biopsies, statistical analysis of complications in men and women of different age groups is believed to be helpful in borderline cases for a better evaluation of risk factors versus the value of the information gained by the procedure. The 95 percent confidence interval and the X2 test showed a significantly lower rate of pneumothorax in mediastinal and pleural based lesions as compared to intralobar changes. Age and sex were found to be the most important parameters for the rate and degree of pneumothorax. The incidence of pneumothorax (mean value 27,2%) steadily increased with age from 2 of 10 in the age group of 9 to 29 years, to 3 to 10 in age group up to 69 years in both sexes, and after 70 years it was as high as 4 of 10. In women the relative frequency of pneumothorax was lower for the respective age groups than in men. Other factors influencing the complications include: experience of the operator, size and depth of the lesion to be punctured and to a certain degree the outer diameter of the needle being used.