Volatile fatty acids (VFA) production by anaerobic fermentation of municipal solid wastes was studied at pilot-plant level. A plug-flow reactor (80 l total volume), with and without concentrated sludge recirculation, was employed to digest an organic fraction of the municipal solid waste (OFMSW)-based substrate. This was composed of OFMSW mechanically-selected in a full-scale plant with OFMSW collected from fruit and vegetable markets and mixed in a way to obtain a feed mixture with a proportion of total volatile solids (TVS)/total solids (TS) of 60%. The fermentation process was studied at different retention times (RT) (from 8 h to 8 days) and at very high organic loading rates (OLR) (from 22.4 to 85.2 kg VS/m3 d) at mesophilic temperature (37 +/- 2-degrees-C). Results showed an increase in VFA production at higher retention times (from 11.8 g/l at 2 days to 23.1 g/l at 6 days RT) when no recirculation was used. VFA production was also high when part of the digested sludge was recirculated (19.5 g/l at 2 days of hydraulic RT and 8 days of solid RT). Acetic and butyric were the acids produced in the largest quantities.