Background Exposure to parental smoking is associated with wheeze in early childhood, but in 2006 the US Surgeon General stated that the evidence is insufficient to infer a causal relationship between exposure and asthma in childhood and adolescents.

Aims To examine the association between maternal and paternal smoking and symptoms of asthma, eczema and rhinoconjunctivitis.

Methods Parents or guardians of children aged 6–7 years completed written questionnaires about symptoms of asthma, rhinoconjunctivitis and eczema, and several risk factors, including maternal smoking in the child's first year of life, current maternal smoking (and amount) and paternal smoking. Adolescents aged 13–14 years self completed the questionnaires on these symptoms and whether their parents currently smoked.

Results In the 6–7-year age group there were 220 407 children from 75 centres in 32 countries. In the 13–14-year age group there were 350 654 adolescents from 118 centres in 53 countries. Maternal and paternal smoking was associated with an increased risk of symptoms of asthma, eczema and rhinoconjunctivitis in both age groups, although the magnitude of the OR is higher for symptoms of asthma than the other outcomes. Maternal smoking is associated with higher ORs than paternal smoking. For asthma symptoms there is a clear dose relationship (1–9 cigarettes/day, OR 1.27; 10–19 cigarettes/day, OR 1.35; and 20+ cigarettes/day, OR 1.56). When maternal smoking in the child's first year of life and current maternal smoking are considered, the main effect is due to maternal smoking in the child's first year of life. There was no interaction between maternal and paternal smoking.

Conclusions This study has confirmed the importance of maternal smoking, and the separate and additional effect of paternal smoking. The presence of a dose–response effect relationship with asthma symptoms suggests that the relationship is causal, however for eczema and rhinoconjunctivitis causality is less certain.