The paper concerns the results of a pilot-scale study of the simulation of the start-up phase of the thermophilic semi-dry anaerobic digestion of the organic fraction of municipal solid wastes. The aim of the study was to aid and shorten the start-up phase of the full-scale plant (500 t/d) in Verona-Ca' del Bue, where the semi-dry anaerobic digestion process is being used. The substrate used in the experimentation was the mechanically sorted organic fraction of municipal solid waste (MS-OFMSW) enriched with the putrescent fraction from the source sorted OFMSW in order to simulate the substrate which is dealt with in the Verona plant. The results of the pilot scale study agreed with literature data and previous work of the authors: it showed a specific gas production of 0.23 m(3)/kgTVS(feed) and a gas production rate of 2.1 m(3)/m(3) d when operating at a specific organic loading rate of 0.135 kgTVS(feed)/ kgTVS(reactor)d. No problems regarding process stability were encountered in the gradual acclimation of the biomass. The design organic loading rate of 9 kgTVSfeed/m(reactor)(3)d was reached in about 30 days, during which the total solids content in the feed was increased. Only a partial comparison with the full scale start-up, which is now in progress, is possible: this shows an initial general concordance with the results found in previous work.