PUMICE-rich pyroclastic flows leave deposits called ignimbrites(1), which are common in the eruption records of are and intracontinental volcanoes(2,3). Pyroclastic flows represent the most dangerous manifestation of volcanism(4) because of their volumes, rapid emplacement at high temperatures, and capacity to extend >100 km from the source. However, maximum distances travelled and extents of areas destroyed by pyroclastic flows remain poorly known. Historic examples have been relatively small, the largest reaching to only similar to 35 km from the vent(5), but prehistoric ignimbrites over 150 km across are known(3,6-10). The largest ignimbrites are invariably partly buried or eroded, and only minimum estimates of their size (and hence of their destructive capacity) can be made. Here we describe the similar to 1-Myr-old Kidnappers ignimbrite, erupted from the Taupo Volcanic Zone in New Zealand, which we have correlated for greater than or equal to 385 km and which thus represents the most widespread ignimbrite yet known, We suggest that this wide distribution primarily reflects a coupling of high initial flow velocities with large mass fluxes, and that favourable flow paths and emplacement of flows over a wet substrate may have aided their mobility.