It seems to be generally accepted that genetic factors take a place among the causes of coronary heart disease. Careful and extensive family studies, among others by workers in Carter's group, have shown that the risk to close relatives of patients with coronary heart disease is significantly increased. This may, however, be an effect of the common social background in families. Systematic twin investigations have been needed, which could distinguish between the effects of common environment and common genes, and allow an estimate of the relative importance of genetic and environmental factors.

The present study is an approach to this problem by using twins with coronary heart disease in the Danish Twin Register (Hauge et al, 1968). This was founded in 1954 and has been developed ever since. It now contains total medical information on about 10 000 unselected pairs of twins born in Denmark in the period 1870-1910, where both partners have survived the age of five. On January 1, 1968, coronary occlusion had been registered in a total of 352 twins.