Pyroclastic density currents formed through collapse of eruptive fountains commonly have runout distances of the order of tens of kilometers. A possible cause of this high flow mobility is elevated interstitial pore gas pressure, which may have various origins. We investigated experimentally the generation of pore pressure at the impact zone of an eruptive fountain, where concentrated pyroclastic density currents emerge from compaction of a free falling gas-particle mixture. We simulated pyroclastic fountain collapse by releasing glass beads of mean sizes of 29–269 μm from a hopper at height of 3.27 m above a 5 m-long horizontal channel, and we measured pore air pressure in the impact zone. During free fall, the granular mixtures accelerated and expanded to reach particle concentrations of 1.6–4.4 vol.% before they impacted the base of the channel. Upon impact, the particles accumulated to form concentrated granular flows with particle concentrations of 45–48 vol.% and pore air pressures that indicated almost full weight support for particle sizes ≤76 μm. Both the amount of pore pressure in the impact zone and the flow runout distance increased as we decreased the particle size and hence the hydraulic permeability of the concentrated granular mixtures. Our results suggest that pore gas pressure in concentrated pyroclastic density currents can be generated at the impact zone of collapsing fountains and that small particle size conferring low permeability and long pore pressure diffusion timescale is one of the main causes of long flow runout distances.