Between 1984 and 1987 there were 7 cases of sudden cardiac death during organized mass runs in Switzerland, and between 1978 and 1987 there were 3 cases during the nine largest mass running events (total 8 cases of sudden death during the race). Based on numbers of participants in all events 1984-1987, or in the nine largest events 1978-1987, an incidence of 1 sudden death per 129,500 hrs. of running (95% confidence interval 1/62,500-1/263,000 hrs.), or 1 sudden death per 117,000 hrs. of running (1/45,000-1/311,000 hrs.) respectively, was estimated. This estimate is higher than the rate of 1 sudden death per 396,000 hrs. of noncompetitive jogging found in a study from the United States (Thompson et al.: J. Amer. med. Ass. 1982; 247: 2535-2538). The Swiss incidence of sudden cardiac death during organized mass runs was 50 to 1000 times higher than the incidence expected by chance alone (as estimated from national death register data). All 8 cases of the study were men, the younger four aged 23 yrs. on average (range 20-31 yrs.), the older four aged 49 yrs. (46-53 yrs.). Autopsy in three of the younger men identified hypertrophic cardiomyopathy in one instance whereas in the two other cases no plausible cause of death could be found. The two autopsies performed in older men both showed severe coronary heart disease. Only in 1 case out of the 8 were possible prodromal symptoms of the subsequent death, such as fatigue and nausea, observed, and the average prevalence of known cardiovascular risk factors was low. None of the 8 runners dying suddenly was completely untrained, but 6 out of 8 had only modest running experience, i.e. a low number of years of running. This study confirms that there is probably a clearly increased risk of sudden death during running events with a competitive character, but this acute elevation of risk should probably not be overstated in view of both its very low population - attributable risk and the important potential of regular exercise for overall coronary risk reduction and health promotion.