Borexino is a large-volume liquid scintillator detector placed in the underground halls of the Lab-oratori Nazionali del Gran Sasso in Italy. It is able to detect in real time neutrino interactionsbelow 2 MeV, due to the very high radio-purity reached by the detector. The interaction rate ofthe 0.862 MeV7Be neutrinos is 49±3(stat)±4(syst) counts/(day·100 ton) , in agreement with theoscillation hypothesis in the MSW Large Mixing Angle scenario. Our result is the first direct mea-surement of the survival probability for solarνein the transition region between matter-enhancedand vacuum-driven oscillations.