<p>In this study we present a novel general methodology for Probabilistic Volcanic Hazard Assessment (PVHA) for lahars. We apply the methodology to perform a probabilistic assessment in the Campanian Plain (Southern Italy), focussing on syn-eruptive lahars from a reference-size eruption from Somma-Vesuvius. We take advantage of new field data relative to volcaniclastic flow deposits in the target region and recent improvements in modelling lahars. We explore the effect of uncertainty on the flow initial conditions on the invasion of lahar in the target area, by sampling coherent sets of values for the input model parameters and performing a large number of simulations. We analyse the simulation outputs to produce hazard curves, hazard maps and probability maps for the maximum flow thickness, and hazard surface and probability maps for joint thresholds in flow thickness and dynamic pressure. We believe the latter hazard products represent, on one hand, a novel product in PVHA for lahars, and, on the other hand, a useful means for impact assessment.</p>