Treatability of the sanitary young landfill leachate in a pilot-scale anaerobic fluidized bed reactor during reactor startup and steady-state phases was investigated. All runs were carried out at 35 C due to environmental conditions in the mesophilic anaerobic fluidized bed reactor (AFBR) while organic loading rate (OLR) was increased from 2.5 to 37 gCOD/L-day during the 220 days of operation. The AFBR process attained steady-state conditions about on day 80 and a good and stable COD removal were achieved at about 90%. Biogas production in the bed continuously increased during the process. The mean specific biogas production was found 0.52 L biogas/gCOD(rem), while the methane content was about 75%. The attached biomass concentration, measured as volatile solids rapidly increased as containing about 90% of the total biomass concentration. Furthermore, an increase in the suspended solid concentration was found as an evidence of biomass detachment from the media.