The performances and stabilities of three laboratory-scale semi-batch anaerobic digesters, operated at 35 degrees C, treating organic fraction of municipal solid waste (OFMSW) and primary sewage sludge (PSS) mixed at various ratios were compared during the start-up period. The OFMSW:PSS ratios in the feeds to the three digesters were 100:0, 80:20, and 60:40 on a total solids (TS) basis and their organic loading rates (OLR) were 5.2 kg VS m(-3) d(-1), 3.9 kg VS m(-3) d(-1), and 3.2 kg VS m(-3) d(-1), respectively. There were no indications of failure in any of the digesters. However, addition of primary sewage sludge significantly decreased the imbalances observed during the start-up and improved the process performance. The best performance in terms of specific gas production rate (SGPR) was obtained with a feedstock having an OFMSW:PSS ratio of 80:20. The use of inoculum from an operating anaerobic digester enabled a quick start-up and the reactors were capable of handling high OLRs from the beginning of the experiment.