Improvements in analytical design procedures developed over the past ten years have led to a better understanding of the significance of calculated stresses as described in Section l – Design Considerations of the PVRC Interpretive Report of Pressure Vessel Research. On the basis of those stress considerations, the present report estimates, insofar as possible, the factors insuring against the various stress-dependent modes of pressure-vessel failures. For these modes of failure, safety indexes are estimated for present Code allowable design stresses and for higher allowable design stresses with particular emphasis on high strength steels (tensile strengths up to 130,000 psi) and on designs based on yield strength rather than tensile strength.Although the expected change in safety indexes for higher design stresses are presented, no changes in the allowable design stresses are proposed or recommended because such changes are the prerogative and responsibility of Code-making bodies.