A study was undertaken to evaluate the effectiveness of the blood patch technique in the prevention of pneumothorax after transthoracic needle aspiration biopsy. A total of 140 needle biopsies were performed with a coaxial system. Two groups of patients were defined according to whether or not autologous blood was injected into the introducing needle as it was withdrawn after needle aspiration biopsy. Fifty-two biopsies were performed with the blood patch technique (group A), while 88 biopsies were performed without the blood patch technique (group B). The frequency of postbiopsy pneumothorax was 28.8% (15 of 52 patients) in group A and 34.1% (30 of 88 patients) in group B. Chest tube insertion was required in 7.7% (four of 52 patients) in group A and in 9.1% (eight of 88 patients) in group B. There was no statistically significant difference in pneumothorax rate and chest-tube insertion rate between the two groups (P greater than .05). In this series of 140 biopsies, the blood patch technique failed to affect the rate of pneumothorax after transthoracic needle aspiration biopsy.