The application of some fracture mechanics concepts to the prediction of the strength of pressure vessels in which cracks have penetrated entirely through the vessel wall is examined. The strength of cylindrical pressure vessels (with longitudinal through-cracks) is related to the geometry of the cylinder, the crack length, the material yield strength, and fracture toughness. An expression for the crack tip stress intensity factor is derived by considering the influence on the stress near the crack tip of membrane stressing in the cylinder and of bending caused by bulging of the partially constrained material on either side of the crack. The form of the derived expression is verified and empirical constants evaluated from burst data at -320 and -423 F of aluminum alloy (2014-T6 A1) and extra-low-interstitial (ELI) titanium alloy (5A1-2. 5Sn-Ti) The analysis, basdd on the Irwin fracture theory, provides a suitable means for the correlation of the strength of flat-sheet specimens with that of cracked pressurized cylinders. The correlation suggests that the fracture strength of through-cracked cylinders of any radius, crack length, and thickness can be determined from material toughness, yield strength, and a bulge coefficient. Neglect of the effect of pressure bulging in the fracture analysis of pressurized cylinders can cause a significant overestimate of the fracture strength.