The acute effects of earthquake-related psychological stress on total and cardiac mortality were investigated using mortality data from the city of Thessaloniki (Greece) which was hit on 19 and 20 June 1978, by two strong earthquakes, measuring 5.2 and 6.4 degrees on the Richter scale, respectively. During the a priori specified three-day period of 19, 20 and 21 June, mortality from cardiac and 'all other' pathological causes increased significantly. The estimated relative risks (and 95% confidence intervals) were 3.0 (1.5-5.9) for atherosclerotic heart disease, and 1.6 (1.1-2.3) for 'all other' diseases (excluding external causes) as underlying conditions. Stress-related cardiac deaths (proximate cause) were more common when atherosclerotic heart disease was the underlying condition (p approximately 0.10), but there was no evidence of any age or sex predilection.