A catastrophic lahar began on 30 October 1998, as hurricane precipitation triggered a small flank collapse of Casita volcano, a complex and probably dormant stratovolcano. The initial rockslide‐debris avalanche evolved on the flank to yield a watery debris flood with a sediment concentration less than 60 per cent by volume at the base of the volcano. Within 2·5 km, however, the watery flow entrained (bulked) enough sediment to transform entirely to a debris flow. The debris flow, 6 km downstream and 1·2 km wide and 3 to 6 m deep, killed 2500 people, nearly the entire populations of the communities of El Porvenir and Rolando Rodriguez. These ‘new towns’ were developed in a prehistoric lahar pathway: at least three flows of similar size since 8330 ¹⁴C years bp are documented by stratigraphy in the same 30‐degree sector. Travel time between perception of the flow and destruction of the towns was only 2·5–3·0 minutes. The evolution of the flow wave occurred with hydraulic continuity and without pause or any extraordinary addition of water.

The precipitation trigger of the Casita lahar emphasizes the need, in volcano hazard assessments, for including the potential for non‐eruption‐related collapse lahars with the more predictable potential of their syneruption analogues. The flow behaviour emphasizes that volcano collapses can yield not only volcanic debris avalanches with restricted runouts, but also mobile lahars that enlarge by bulking as they flow. Volumes and hence inundation areas of collapse‐runout lahars can increase greatly beyond their sources: the volume of the Casita lahar bulked to at least 2·6 times the contributing volume of the flank collapse and 4·2 times that of the debris flood. At least 78 per cent of the debris flow matrix (sediment < −1·0Φ; 2 mm) was entrained during flow. Copyright © 2004 John Wiley & Sons, Ltd.