An anaerobic digestion of municipal solid waste (MSW) was carried out in a pilot-scale reactor to study anaerobic fermentation as it happens in a sanitary landfill. The reactor was not inoculated and the MSW used was taken from the COGERSA landfill site in Asturias, Spain. The experiments were performed in an opaque PVC reactor at 36 +/- 1 degrees C. Water was added in the same proportion as the daily rainfall at the landfill sire. The pH of the leachates was low (5.5-6.1 approximately) from days 40 to 170 when it began to increase to neutral. 2.25 litres of the leachates produced (previously treated with NaOH and NaHCO3 until day 300) were recirculated from day 120 to the end of the experiment Leachate chemical oxygen demand decreased with time, reaching levels of approx 2000 mg/l by the end of this study. Volatile fatty acids presented a similar behaviour: The main metals found in the leachates were Fe, Mn and Zn. The methane percentage reached a maximum value of 66%.