This study investigated the start-up and treatment efficiency in anaerobic co-digestion of landfill leachate and septage by a UASB process. A UASB reactor was operated at 35 +/- 1 degrees C. Septage and leachate were mixed in the ratios of 3:1, 2:1 and 1:1 on the basis of chemical oxygen demand (COD) and the mixture ratios of the feeds were changed during the experiment. At hydraulic retention time of 1.5 days and an organic loading rate of 6.73 kg COD/m(3)-day (mixture ratio 1:I), the removal efficiencies of total COD, soluble COD, total solids, volatile solids, total volatile fatty acid, total phosphorus, ammonia nitrogen, carbohydrate and protein were 42.2%, 58.1%, 45.3%, 68.2%, 73.4%, 44.3%, 47.8%, 53.7% and 44.4%, respectively. Based on values of the organic loading rate and treatment efficiencies, which are comparable to reported values for wastewater or sewage, it was concluded that a UASB reactor could successfully co-digest leachate and septage. A comparison between the present results and those of a CSTR reactor fed on the leachate-septage mixture was provided.