The Danish Twin Register, which comprises all twins born in Denmark between 1870 and 1920, has been used for studies of selected problems in the field of epidemiology and public health. This report gives a survey of the preliminary results of an investigation of this type, which intends to amplify the present knowledge about the possible harmful effects of smoking on health, by taking advantage of a twin material which makes it possible to keep the genetic factors under control.

A series of 762 MZ and same-sexed DZ twin pairs, which had their smoking habits mapped in 1959, has been followed continuously, and the mortality which has occurred in this group has been analysed in relation to tobacco consumption. Neither MZ nor DZ pairs showed any tendency to increased mortality in the heavier smoking cotwins. When the material was subdivided according to cause of death, no conspicuous exception from the general pattern was found, but the subgroups were relatively small.

The distribution of selected diseases and symptoms, mainly of the cardiovascular and respiratory systems, was studied in this and a second series, followed since 1966, with a total of 1584 pairs.