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Advanced Trauma Life Support guidelines recommend the use of a cannula 3 to 6 cm long to perform needle thoracocentesis for life-threatening tension pneumothorax. The chest wall thickness in the 2nd intercostal space, mid-clavicular line, was determined by ultrasound in 54 patients aged 18 to 55 years, and ranged from 1.3 to 5.2 cm (mean 3.2 cm). In thirty-one patients (57 per cent) the chest-wall thickness (CWT) was greater than 3 cm, the minimum recommended cannula length, although in only two (4 per cent) was it greater than 4.5 cm, the length of cannula commonly used in the UK. As a 3 cm cannula would fail to reach the pleural cavity in over half of patients, we suggest that the recommended shortest length be increased to 4.5 cm. Unsuccessful needle thoracocentesis using a 4.5 cm cannula should be followed immediately by insertion of a longer cannula or a definitive chest drain.