Refuse in landfills becomes stabilized or aged, as organic matter in the refuse gradually degrades and as the soluble inorganic substances dissolve during its long-term stabilization process. Within this paper, this process is referred to as mineralization and the resultant stabilized or essentially stabilized refuse is referred to as "aged refuse."The aged refuse contains a wide spectrum and large quantity of microorganisms, which have a strong decomposition capability for refractory organic matter present in some wastewaters, such as leachate. In this study, aged refuse excavated from two to ten year old closed landfill compartments in Shanghai Refuse Landfill (SRL) was characterized in terms of particulate distribution by screening, and a biofilter consisting of ten year old aged refuse was then used for biofiltration of leachate sampled from the landfill. Typically, 400 kg of screened aged refuse with limiting diameter less than 15 mm was used as biofiltration materials in a round shaped biofilter with 80 cm inner diameter and 80 cm height. Leachate with initial chemical oxygen demand (COD), biological oxygen demand (BOD), and NH3-N concentrations of 3,000-7,000, 540-1,500, and 500-800 mg/L, respectively, was passed through the biofilter. As a result, the corresponding concentrations in the effluent were reduced to lower than 100-350, 10-200, and 10-25 mg/L, respectively, 90-99% removal for these parameters at a hydraulic load of 80-200 L/m3 refuse/day. The color of the effluent became slightly gray, in comparison with the heavy brownish color of the influent. The treatment efficiencies heavily depend on hydraulic load, BOD/COD ratios in the leachate, and preliminary treatment of the aged refuse. A variety of leachates with various BOD/COD ratios was tested. It was found that the effluent deteriorated when BOD/COD ratios were lower than 0.1-0.2. Increase of hydraulic load resulted in a decrease of removal efficiencies. Removal of stone, plastics, and glass, etc., from the aged refuse improved the treatment. A pilot test was conducted at SRL and the experimental results obtained at laboratory scale were verified.