The cavitation which causes severe erosion of hydraulic machine runners is produced by the development of fixed cavities attached at the leading edge of the blades. It is clearly shown, from a comparison between the observations of cavity patterns during model tests and the damage found in the prototype, that the damaged areas of the blades extend in the wak:e of the fixed cavities. In this paper we give experimental evidence that this damage is due to the collapse of transient cavities in the fixed cavity closure region. Visualizations, pressure measurements and LDA flow field measurements using the IMHEF High-Speed Cavitation Tunnel allow us to describe the generation and the dynamics of these transient cavities where the Reynolds number plays a leading role. Moreover the investigation carried out with a special facility, the Cavitation Vortex Generator, reveals that the spherical shock-waves emitted in the last stages of cavity collapse are strong enough (20 kbar) to damage any industrial alloy. These experimental results should be confirmed in order to provide a physical basis for numerical simulation of the fixed cavity development and the generation-collapse cycle of the related transient cavities.