Background There is increasing evidence that contact with other smokers, particularly in the family, is a strong determinant of risk of smoking uptake. A systematic review and meta-analysis of the magnitude of these effects is reported.

Methods Studies were identified by searching four databases to March 2009 and proceedings from international conferences. Meta-analyses were performed using random effects, with results presented as pooled ORs with 95% CIs.

Results 58 studies were included in the meta-analyses. The relative odds of uptake of smoking in children were increased significantly if at least one parent smoked (OR 1.72, 95% CI 1.59 to 1.86), more so by smoking by the mother (OR 2.19, 95% CI 1.73 to 2.79) than the father (OR 1.66, 95% CI 1.42 to 1.94), and if both parents smoked (OR 2.73, 95% CI 2.28 to 3.28). Smoking by a sibling increased the odds of smoking uptake by 2.30 (95% CI 1.85 to 2.86) and smoking by any household member by 1.92 (95% CI 1.70 to 2.16). After adjusting for overestimation of RRs it is estimated that, in England and Wales, around 17 000 young people take up smoking by the age of 15 each year as a consequence of exposure to household smoking.

Conclusions Parental and sibling smoking is a strong and significant determinant of the risk of smoking uptake by children and young people and, as such, is a major and entirely avoidable health risk. Children should be protected from exposure to smoking behaviour, especially by family members.