OBJECTIVE to evaluate the factors that could effect the risk of pneumothorax in patients undergoing transthoracic biopsy. METHODS variables that could increase the risk of pneumothorax were evaluated in 453 CT-guided transthoracic biopsies. Factors were evaluated in two groups: (1) lesion related (presence of emphysema around the lesion, lesion depth, cavitation, presence of fissure/atelectasis and pleural tag in the needle trajectory); and (2) procedure related (biopsy type, needle size, number of passages, level of experience of the operator). All variables were analysed by chi2 test and multivariate logistic regression statistics. RESULTS pneumothorax was developed in 85 (18.8%) out of 453 procedures. A chest tube was inserted in ten (11.7%) of them. Variables that were significantly associated with an increased risk of pneumothorax were depth of the lesion (P<0.001) and severity of the emphysema (P<0.01). CONCLUSIONS the length of the lung parenchyma traversed during the biopsy is the predominant risk factor for pneumothorax in patients undergoing CT-guided transthoracic biopsy. The risk of pneumothorax was also increased with the severity of the emphysema around the lesion.