When in 1905 Gustav Mahler went back home from Trieste after having conducted a concert there, he left a score of his Fifth Symphony filled with autograph corrections. As the Fifth Symphony is the most retouched work of Mahler's, the Trieste score is one of the precious testimonies of the long process of reorchestrating and refining that was stopped only when the composer died. Mahler's 1905 visit to Trieste was his first visit to an Italian cultural territory in quality of conductor and composer. It happened in a specific social context in which a group of illuminated intellectuals was trying to save the cultural life in the city. He came back once again, in 1907, after a visit to Rome where he conducted two concerts and in 1910 lived the biggest disaster in his entire career. Trieste, however, was different, so the Fifth Symphony he conducted there did not pass unnoticed. The study of the score that Mahler left in Trieste shows some important details and corrects the widespread opinion that its role in the genesis of the final version of the Fifth is hardly relevant. Moreover, the set of parts used for the concert in Trieste reveals which of the corrections were actually performed. The comparison of the score used by Mahler with the final version of the Fifth proves the former to be quite significant, as most of the modifications survived all subsequent revisions.