Family size and smoking during pregnancy were studied as mediating factors for social and ethnic variation of lower respiratory tract infection (LRI) in hospital discharge data. The study population consisted of all children aged 0-4 years in the three largest metropolitan areas of Sweden during 1990-94. Maternal smoking during pregnancy increased the risk of children being admitted to hospital for LRI during their first 3 years of life, with an adjusted odds ratio (OR) of 1.3 for the age-group 0-1 years. The risk attributed to smoking during pregnancy was the same in children of mothers in ethnic groups in which smoking during pregnancy was related to social adversity as in those in which it was not. Having at least one sibling increased the risk of being admitted to hospital for LRI in the age group 0-1 years (adjusted OR 2.2). This risk was lower in children in families in which the mother was born in southern Europe, Africa, Asia or Latin America, suggesting a contextual relation to ethnicity for this risk factor. It is concluded that family size and smoking during pregnancy are important mediators of the risk for LRI related to social adversity and ethnicity in Swedish children below 2 years of age.