Background Most occurrences of presumed triggers of acute myocardial infarction (AMI) do not actually trigger an AMI. The relation between vulnerability to development of AMI and timing of triggers of AMI is poorly understood. The 1989 Loma Prieta earthquake struck the San Francisco Bay Area at 5:04 PM during a period of relatively reduced circadian vulnerability to AMI. The 1994 Northridge earthquake awakened citizens of Los Angeles at 4:31 AM, also during a period of otherwise reduced vulnerability to AMI, but superimposed the stress of the earthquake upon the known stress of awakening. Study of times of onset of natural disasters and their effect on precipitation of AMI may provide insight into the physiology of triggering. This study was designed to compare the triggering ability of 2 similarly intense earthquakes that occurred singly or superimposed upon another trigger. Methods and Results California discharge records were analyzed from the week before, the day of, and the 6 days after the Loma Prieta and Northridge earthquakes and the same dates of control years for total admissions, AMI admissions, and congestive heart failure admissions. There was no statistically significant increase in AMI admissions in the San Francisco Bay Area on the day of the Loma Prieta earthquake compared with the days before or after the earthquake or compared with the same day in 1990. In contrast, there was a 110% increase in admission rate for AMI in Los Angeles County on the day of the Northridge earthquake compared with the average of the 7 days before the earthquake (z = 4.349, P < .001). Similarly, there was a >80% increased risk of admission for AMI on the day of the Northridge earthquake in 1994 compared with the same date in 1995 (relative risk 1.828, 95% confidence intervals 1.289 to 2.592). Conclusions Extreme emotional stress superimposed on the stress of awakening results in enhanced triggering of AMI. When emotional stress occurs at less vulnerable periods, there is no significant increase in incidence of AMI. (Am Heart J 1999;137:830-6.)