For the first time a physical model, constrained by monitoring data, is used to derive a quantitative estimate of the evolution in time of magmatic gases that enter a hydrothermal system of an active volcano. The site is Campi Flegrei (west of Naples, in Italy), a caldera that had a large ground inflation in 1982–1984 followed by 20 yr of subsidence. More recently the behavior of the system has changed: the magmatic component of fumaroles has increased, swarms of earthquakes are more frequent, and the ground has started a general uplifting trend, indicating that the hydrothermal system undergoes repeated injections of magmatic fluid. Physical simulations of the process show that total injected fluid masses are the same order of magnitude as those emitted during small to medium size volcanic eruptions, and their cumulative curve highlights a current period of increasing activity. Gas emission studies coupled with physical modeling can be extremely effective in predicting magmatic evolution and eruptive activity at volcanoes.