The Envirex granular activated carbon biological fluidized bed (GAC-BFB) process was used to treat landfill leachate containing organic material with typical COD values in the range 800 to 2,700 mg/l, but with high concentrations of ammonia in the range 220 to 800 mg/l. Prior to treatment the leachate was softened by raising the pH to 9.0 using lime in order to remove any contaminating heavy metals. Two identical pilot-plant GAC-BFB's were used in series to treat the softened leachate. The first of these was intended to achieve carbonaceous removal whilst the second was used for ammonia removal by nitrification. During the first four months of operation at a loading rate of 1.56 kg ammonia-N/m3 d, the reactors removed 70% of the ammonia, 60% of COD and achieved a complete removal of BOD. After optimising the reactor operating regime, it proved possible to achieve greater than 90% removal of ammonia at a loading rate of 0.71 kg N/m3 d; however the COD removal remained unchanged. In order to compare the ammonia removal efficiency of the GAC-BFB system, two conventional activated sludge reactors were operated in parallel at hydraulic retention times of 24 and 36 hours (equivalent to ammonia loadings of 0.72 and 0.48 kg ammonia-N/m3 d) and with a sludge age of 20 days. These reactors were not able to nitrify and although a BOD removal of 90% was achieved the COD removal was poor at 20%. Thus the GAC-BFB system offers a highly effective option for the biological removal of ammonia from high ammonia leachates, with the additional advantage of a good COD removal.