To examine the association of current smoking behavior of adolescents and young adults with the smoking behavior of their parents, siblings, friends and spouses.Using survey data from a large twin-family sample, the association between the smoking behavior of participants and that of their family members, friends and spouses was investigated by calculating the relative risk. To disentangle sex and age differences, calculations were carried out separately for males and females and for three different age groups: 12-15, 16-20 and 21-40 years old.The smoking behavior of the participants was significantly influenced by the smoking behavior of parents, siblings and friends, but all relative risks decreased with age. No differences in relative risk were found between having older or younger smoking siblings. Within each age group, the relative risk to smoke when having a smoking friend was comparable to the relative risk to smoke when having a smoking same-age and same-sex sibling. For the older participants, the relative risk to smoke was higher for monozygotic (MZ) twins with a smoking co-twin than for dizygotic (DZ) twins with a smoking co-twin. Most findings were sex-dependent: same-sex smoking family members influenced smoking behavior more than opposite-sex family members. The significant association of the smoking behavior of spouses decreased with age, which suggests that assortment for smoking is based on similarity at the time dating began.The results highlight the importance of both social and genetic influences on smoking behavior, with genetic influences increasing with the age of the participant.