[en] A simple probabilistic methodology is used to investigate the benefit, in terms of reduction of disruptive failure probability, which comes from the application of periodic in-service inspection (ISI) to nuclear pressure vessels. The analysis indicates the strong interaction between inspection benefit and the intrinsic quality of the structure. In order to quantify the inspection benefits, assumptions are made which allow the quality to be characterized in terms of the parameters governing a Log Normal Distribution of time - to - failure. Using these assumptions, it is shown that the overall benefit of ISI is unlikely to exceed an order of magnitude in terms of reduction of disruptive failure probability. The method is extended to evaluate the effect of the periodicity and reliability of the inspection process itself