Collapse of Santa Ana volcano during the late Pleistocene produced the voluminous and extremely mobile Acajutla debris avalanche, which traveled ∼50 km south into the Pacific Ocean, forming the broad Acajutla Peninsula. The subaerial deposit covers ∼390 km²; inclusion of a possible additional ∼150 km² submarine component gives an estimated volume of 16 ± 5 km³. Hummocks are present to beyond the coast-line but are most prominent in four clusters corresponding to the location of buried bedrock ridges. Bulking in distal portions incorporated accessory Tertiary-to-Quaternary volcaniclastic rocks and ignimbrites. Modern Santa Ana volcano was constructed within the collapse scarp, visible only on its northwest side, following an apparent transition in eruptive style. More than 286,000 people, the country's main port, and important agricultural land now overlie the Acajutla debris-avalanche deposit, which is one of only a few in Central America to exceed 10 km³ in size. Because major edifice failures are high-impact, low-frequency events, the probability of a future Acajutla-scale collapse is very low. However, a collapse even an order of magnitude smaller in volume from modern Santa Ana volcano would impact heavily populated areas. The Acajutla failure was perpendicular to a NW-trending fissure system cutting across Santa Ana volcano, which may also influence future failure orientations. The current structure of Santa Ana volcano suggests that future collapses are most likely to the southwest, but the possibility of northward failures cannot be excluded.