[en] At present the significance of a defect found during service is assessed by deterministic means, for example those given in Section XI of the American Society of Mechanical Engineers (ASME) Code. It is proposed that by the use of probabilistic fracture mechanics the suitability of a vessel for continued service may be assessed, by comparing the calculated probability of failure for the defect with a design figure for the vessel. An analytical method is given which allows the probability of failure for a given defect indication to be calculated, with allowance being made for the sensitivity of the detection method, defect growth, service loading, and variation of material properties. Examples are presented which show the application of the methodology. The extension of the analysis to include other failure modes is discussed. (author)