We examined the hypothesis that foetal exposure to maternal passive smoking is associated with childhood asthma, allergic rhinitis, and eczema. METHODS. The study was a population-based cross-sectional survey of Hong Kong Chinese children aged ≤14 years carried out in 2005 to 2006. RESULTS. Foetal exposure to maternal passive smoking was significantly associated with wheeze ever (OR 2.05; 95% CI 1.58-2.67), current wheeze (OR 2.06; 95% CI 1.48-2.86), allergic rhinitis ever (OR 1.22; 95% CI 1.09-1.37), and eczema ever (OR 1.61; 95% CI 1.38-1.87). Foetal exposure to maternal active smoking was significantly associated with asthma ever (OR 2.10; 95% CI 1.14-3.84), wheeze ever (OR 2.46; 95% CI 1.27-4.78), and current wheeze (OR 2.74; 95% CI 1.24-6.01) but not with allergic rhinitis ever (OR 1.01; 95% CI 0.70-1.46) or eczema ever (OR 1.38; 95% CI 0.87-2.18). The dose response relationship between wheeze ever and current wheeze with increasing exposure, from no exposure to maternal passive smoking and then to maternal active smoking, further supports causality. CONCLUSION. There is significant association between foetal exposure to maternal passive smoking and maternal active smoking with childhood asthma and related atopic illnesses. Further studies are warranted to explore the potential causal relationship.