Campi Flegrei (Phlegrean Fields) is an active volcanic center near Naples, Italy. Numerous eruptions have occurred here during the Quaternary, and repeated episodes of slow vertical ground movement (bradyseism) have been documented since Roman times. Here, we present a quantitative model that relates deformation episodes to magma degassing and fracturing at the brittle-ductile transition in a magmatic-hydrothermal environment. The model is consistent with field and laboratory observations and predicts that uplift between 1982 and 1984 was associated with crystallization of ∼0.83 km3 of H2O-saturated magma at 6 km depth. During crystallization, ∼6.2 × 1010 kg of H2O and 7.5 × 108 kg of CO2 exsolved from the magma and generated ∼7 × 1015 J of mechanical (PΔV) energy to drive the observed uplift. For comparison, ∼1017 J of thermal energy was released during the 18 May 1980 lateral blast at Mount St. Helens.