Anaerobic treatment of the young landfill leachate in a pilot-scale fluidized bed reactor was performed to observe the effects of startup on COD removal, biogas production and biomass attachment. During the experiments, the organic loading rate (OLR) was increased gradually from 2.5 to 27 g COD l(-1) day(-1) with varying the feed rate and the hydraulic retention time in the range of 5.5-13 1 day(-1) and 1-2.35 days, respectively. The anaerobic fluidized bed reactor (AFBR) showed an excellent performance and attained steady state conditions with COD removal of 90% after 80 days. Biogas production in the reactor continuously increased and its average yield (Y(gas)) was defined as 0.531 biogas g(-1) COD(rem)(-1) with a correlation of 99%. Greater microbial attachment was found at the increased loading rates and the attached biomass concentration (X(attach)) consisted of about 90% of the total biomass concentration. In addition, the substrate utilization rate indicated a linear increase as a function of time during the process.