The Landfill of Meruelo (Cantabria, Spain) is situated in a warm area. The leachates are produced at a temperature higher than 17 degrees C, even during the winter period. The treatment of the leachates was carried out in a laboratory-scale upflow anaerobic sludge-bed reactor (UASB) at a temperature of 35 degrees C and at ambient temperatures (15-20 degrees C), with the aim of determining the effect of temperature on the COD removal efficiency. At a temperature of 35 degrees C, the organic loading rate was up to 30 kg COD m(-3) day(-1) and at ambient temperatures organic loading rates, were lower than 15 kg COD m(-3) day(-1). The average performance of COD removal efficiency was always higher than 82% in both cases, at hydraulic retention times shorter than a day. No nutrients were added to the leachates throughout the study. Copyright (C) 1997 Elsevier Science Ltd.