Mortality rates in Denmark from ischemic heart diseases (IHD), other heart diseases and unknown causes are presented for the period 1968-92. In all age groups, mortality from IHD is higher at the beginning of the period than at the end. For other heart disease, the plot of the mortality rate is U-shaped for the age groups 65-84 and > or = 85, but first decreases and is then constant for the age group 30-64. There are an increasing number of deaths from symptomatic heart disease. For the group of unknown causes, the rates are increasing for all sex and age groups. The relationship between deaths from IHD and deaths from unknown causes varies with period, age, sex and region. For women in Copenhagen in the age group 30-64, the mortality rate from unknown cause is higher than the rate for IHD at the end of period. Vital statistics must therefore be used cautiously in analysing trends for IHD, and even the validity of temporal changes within a country must be questioned.