Tension pneumothorax is commonly treated with needle decompression (ND) at the 2nd intercostal space midclavicular line (2nd ICS MCL) but is thought to have a high failure rate. Few studies have attempted to directly measure the failure rate in patients receiving the intervention. We performed a retrospective analysis of 10 years of patients receiving prehospital ND. CT scans were reviewed to record the location of catheters left indwelling and the proportion of patients who did not have any pneumothorax. Chest wall thickness was measured on both injured and uninjured sides at the 2nd ICS MCL and compared with the recommended alternative, the 5th ICS anterior axillary line (5th ICS AAL). We identified 335 patients that underwent prehospital ND who had CT scans performed. Using our two different radiologic methods of assessing failure, 39 per cent and 76 per cent of attempts at ND failed to reach the pleural space. In addition, at least 39 per cent of patients did not have a tension pneumothorax. Injured chest walls were significantly thicker than uninjured chest walls at both the 2nd ICS MCL and the 5th ICS AAL (both P < 0.005.) Increasing chest wall thickness correlated with the failure of the catheter to reach the pleural space. Using an 8-cm catheter at the 5th ICS AAL, iatrogenic cardiac injury was at risk in 42 per cent of patients. This series confirms the high failure rate of ND at the 2nd ICS MCL, but further studies are needed to assure the safety of using larger catheters at the 5th ICS AAL.