To examine the association between maternal smoking and non-syndromic orofacial clefts in infants. A meta-analysis of the association between maternal smoking during pregnancy was carried out using data from 24 case-control and cohort studies. Consistent, moderate and statistically significant associations were found between maternal smoking and cleft lip, with or without cleft palate (relative risk 1.34, 95% confidence interval 1.25-1.44) and between maternal smoking and cleft palate (relative risk 1.22, 95% confidence interval 1.10-1.35). There was evidence of a modest dose-response effect for cleft lip with or without cleft palate. The evidence of an association between maternal tobacco smoking and orofacial clefts is strong enough to justify its use in anti-smoking campaigns.