The aim of this study was to evaluate the performance of thermophilic sludge digestion at decreasing sludge retention time (SRT) and increasing organic loading rate (OLR), in terms of methane production, effluent stabilisation, hygienisation and dewaterability. Focus was put on determining indicators to help prevent process failure. To this end, a lab-scale reactor was operated for nearly 2 years at 55 °C. Methane production rate was increased (from 0.2 to 0.4-0.6mCH43 mreactor- 3 d-1) by decreasing the SRT from 30 to 15-10 days, while increasing the OLR from 0.5 to 2.5-3.5 kg VSmreactor- 3 d-1. Sludge dewaterability was worsened at SRT below 15 days; while pathogen destruction was always successful. The following concentrations might be used to prevent process failure: VFA C2-C5 (3.7 g COD L-1), acetate (0.6 g L-1), acetate/propionate (0.5), intermediate alkalinity (1.8 g CaCO3 L-1), intermediate/partial alkalinity (0.9), intermediate/total alkalinity (0.5), CH4 in biogas (55%). © 2009 Elsevier Ltd. All rights reserved.