Finnish site, due to the absence of information on cigarette smoking history in non-twin siblings (from previous surveys), families with dizygotic co-twins concordant for a lifetime history of regular smoking were prioritized. Some families with monozygotic co-twins concordant for lifetime smoking were also enrolled in the study, but only one monozygotic twin per pair was included for linkage analyses. Priority was given to families of dizygotic twins with histories of heavier cigarette consumption, particularly where the female twin was a regular smoker (in a cohort where smoking by women was rare). On both sites, during the screening interview, sufficient information on lifetime history of cigarette smoking was obtained from the index case to confirm family eligibility, and permission was requested to contact available parents and full biological siblings for participation in the study.
