The purpose of this study was to examine socio-economic differences in the risk of hospitalisation among children aged 0-5 years in Denmark from 1985 to 2004. All children born between 1985 and 2004 (n=1,278,286) were followed for hospital admissions for infectious diseases from the 29th day of life until the children reached the age of 6 years or the end of 2004, whichever came first. Information on parental socio-economic position (education, labour market attachment and household income) was gathered through record linkage with administrative registries. Infections were grouped into upper respiratory, lower respiratory, gastrointestinal, ear and fever infections. The data were analysed using Cox regression. Children of parents on sick leave or early retirement had an increased risk of being hospitalised with an infection compared with children of employed parents. A clear inverse educational gradient in risk of offspring hospitalisation was also found. From 1985 to 2004 the inverse associations between parental education and risk of hospitalisation grew stronger, whereas the comparatively weaker association between household income and risk of offspring hospitalisation decreased in magnitude. The association between socio-economic status and hospitalisation was strongest for lower respiratory, gastrointestinal and ear infections. This study documented a socially patterned hospitalisation of pre-school children in Denmark. Future studies should investigate possible explanations for the increased risk among children from families with low socio-economic status.