Adaptation of biohydrogen producing extreme-thermophilic bacteria to household solid waste (HSW) at extreme-thermophilic temperature (70 degrees C) was investigated. Inocula received from an extreme - thermophilic glucose fermentation reactor were exposed to increasing HSW concentrations from 1 g-VS/L to 10 g-VS/L via repeated batch cultivation. It was found that repeated batch cultivation was a very useful method to adapt and enrich biohydrogen producing mixed cultures that could ferment HSW with high hydrogen yield and without significant lag phase. For unadapted cultures (inocula from simple substrate-glucose to complex substrate - HSW), hydrogen was produced only in the HSW concentration of 1-2 g-VS/L and the lag phase required more than 2 days. After adaptation, hydrogen was produced directly in the HSW feedstock (10 g-VS/L) with the maximum yield of 101.7 +/- 9.1 mL H(2)g VS(adddd). Acetic acid was the main fermentation product in all HSW concentration cultivation. Furthermore, hydrogen production was demonstrated in continuous system with adapted cultures while process failure was found with unadapted cultures. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.