The critical edition of Vivaldi's instrumental works is now enriched with a new title, substantial and interesting: this is the concerto for two violins and two cellos, RV 575, a work previously published in the Ricordi-Malipiero edition and performed and recorded numerous times without its incompleteness ever being noticed: this is a most unusual instance where the loss of a single folio of the manuscript, containing around 14 bars of music, produced no obvious harmonic or melodic hiatus between the surrounding material, thereby giving the impression of a work preserved in its entirety. However, a careful study of the work shows that, on the contrary, there is a significant gap, for the filling of which the editor offers a plausible reconstruction. In addition to this large section of music, the manuscript is seen to lack a few short 'echo' responses between the soloists: one more reason for republishing the work in a critical edition with commentary that is designed to stimulate better-informed performances in the future.