The purpose of this analysis was to study the proportion of deaths in the forensic autopsy service that occur in the absence of any witnesses and what is the incidence of the witnessed sudden and unexpected natural deaths. The material comprised all 799 adult males (25--64 years) autopsied at the Department of Forensic Medicine University of Helsinki in 1976. In 350 (43.8%) cases the deceased was found dead. Of these unwitnessed deaths 205 were due to various non-natural causes whereas 125 were due to diseases. The incidence of unwitnessed natural death in the province of Uusimaa from which the autopsies came was 45.1 and that of witnessed instant (death within 10 minutes) natural death 33.2 per 100 000 of the respective male population, corresponding to 6.2% and 4.5% of all natural deaths of males in this age group. Cardiovascular and especially ischaemic heart diseases comprised the major proportion of all unwitnessed and sudden and unexpected natural deaths, whereas pneumonia was the leading category in the remaining cases. Quite often (17.4%) chronic alcoholism, cirrhosis of the liver or moderate alcohol intoxication were recorded as contributing factors to the unwitnessed and sudden and unexpected natural deaths from cardiovascular and other diseases.