The protective properties of knife-resistant armour are quantified by the distance a test blade penetrates beyond a test sample into clay at a given energy. At present there are two proposed standards: penetration to 5 mm and penetration to 20 mm. Armour made to the higher standard specification (5mm) is necessarily heavier as it offers more protection. To determine the safety of these standards a retrospective review of 71 consecutive computerised tomographic (CT) scans was made. The minimum distance from the skin to the vital organs was measured. No organ would have been breached at 5 mm of knife penetration deep to body armour. 41% of pleurae, 61% of livers, 64% of femoral arteries, 25% of spleens and 6% of hearts would have been breached at a depth of 20 m of knife penetration. There was no significant difference in the minimum skin to organ distances between male and female subjects. The 20 mm standard does not offer adequate protection against knife attacks.