BACKGROUND: The potassium-titanyl-phosphate (KTP) laser technique for photo-selective vaporisation of the prostate (PVP) has been regularly improved over the last decade. Nonetheless, decreasing efficiency of tissue vaporisation during the course of the operation and macroscopic alterations of the laser fibre's tip are regularly observed and seem to affect the outcome of this procedure. OBJECTIVE: To investigate the course of power output and to determine the type and extent of fibre deterioration during PVP. DESIGN, SETTING, AND PARTICIPANTS: Forty laser fibres were investigated during PVP in 35 consecutive patients with prostatic bladder outflow obstruction between January 2007 and August 2007 in a university hospital. INTERVENTION: All patients underwent PVP performed by three different surgeons using the 80-W KTP laser. MEASUREMENTS: Power output was measured at the beginning and regularly throughout PVP and throughout in vitro vaporisation without fibre-tissue contact. Microscopic documentation of the fibre tip was performed after the procedure. RESULTS AND LIMITATIONS: Carbonisation and melting of the fibre tip was regularly visible and appeared to be more pronounced as more energy was applied. Additionally, 90% of the fibres showed a significant decrease of power output during PVP, resulting in an end-of-lifespan (ie, 275-kilojoule) median power output of 20% of the initial value. Final median power output after in vitro vaporisation was 83% of the starting value. The extent of the structural and functional changes might only be valid for the operative technique performed in this investigation. CONCLUSIONS: Fibre deterioration caused significant reduction of power output during PVP. This finding is an explanation for the often observed decreasing efficiency of tissue ablation and may also be responsible for some of the typical drawbacks and complications of PVP. Hence, improvements in fibre quality are necessary to advance the efficiency of this technique.