This paper examines the operating parameters and treatment efficiency in digestion of septage with landfill leachate. Four lab-scale CSTR digesters were operated at 35 +/- 1 degrees C. Septage and leachate were mixed at the ratio of 1:0, 1:1, 2:1 and 3:1 on the basis of chemical oxygen demand (COD). For each digester, the solids retention times (SRT) were controlled at 20, 10, and 5.3 days. The organic loadings and volatile solids loadings ranged from 0.3 similar to 1.4 kg COD/m(3)/day to 0.79 similar to 3.7 kg/m(3)/day, respectively. For the same SRT, a higher ratio of septage increased the removal efficiencies of total GOD, ammonia nitrogen and total phosphorus (T-P). The presence of septage enhanced the decrease of the NH3-N concentration of the leachate by 40%. When the organic loading was increased to 0.315 kg COD/m(3)/day (septage/leachate = 3:1) based on SRT 20 days, the removal efficiencies of total COD, NH3-N, T-P, volatile solids, carbohydrate and protein were 86, 69, 86, 28, 82 and 80%, respectively. The methane yield increased with an increasing septage fraction. The dewaterability of the digested sludge was also determined.