The threat of a great earthquake has compelled California to develop a disaster plan for catastrophic medical events that calls for local response with state-coordinated mutual aid and casualty evacuation, if necessary. During the 1989 Loma Prieta earthquake that killed 63 people and injured 3,700, local emergency medical services systems were busy but not stressed excessively. The medical mutual aid system delivered medical personnel, supplies, and blood. One hospital suffered severe nonstructural damage, but it was able to treat large numbers of casualties. Our system performed admirably in this limited response, but was hampered by difficulties with disaster intelligence, communications, emergency medical services dispatch, patient care records, hospital damage, and inadequate disaster training. We describe our state's mutual aid system, the Loma Prieta response, and lessons and recommendations for the future.