The January 1997 East Rift Zone eruption on Kilauea volcano, Hawaii, occurred within a network of continuous Global Positioning System (GPS) receivers. The GPS measurements reveal the temporal history of deformation during dike intrusion, beginning ∼8 hours prior to the onset of the eruption. The dike volume as a function of time, estimated from the GPS data using elastic Green's functions for a homogeneous half-space, shows that only two thirds of the final dike volume accumulated prior to the eruption and the rate of volume change decreased with time. These observations are inconsistent with simple models of dike propagation, which predict accelerating dike volume up to the time of the eruption and little or no change thereafter. Deflationary tilt changes at Kilauea summit mirror the inferred dike volume history, suggesting that the rate of dike propagation is limited by flow of magma into the dike. A simple, lumped parameter model of a coupled dike magma chamber system shows that the tendency for a dike to end in an eruption (rather than intrusion) is favored by high initial dike pressures, compressional stress states, large, compressible magma reservoirs, and highly conductive conduits linking the dike and source reservoirs. Comparison of model predictions to the observed dike volume history, the ratio of erupted to intruded magma, and the deflationary history of the summit magma chamber suggest that most of the magma supplied to the growing dike came from sources near to the eruption through highly conductive conduits. Interpretation is complicated by the presence of multiple source reservoirs, magma vesiculation and cooling, as well as spatial variations in dike-normal stress. Reinflation of the summit magma chamber following the eruption was measured by GPS and accompanied a rise in the level of the Pu'u O'o lava lake. For a spheroidal chamber these data imply a summit magma chamber volume of ∼20 km³, consistent with recent estimates from seismic tomography. Continuous deformation measurements can be used to image the spatiotemporal evolution of propagating dikes and to reveal quantitative information about the volcanic plumbing systems.