Introduction: To present the work previously and presently being carried out based on the nationwide Childcare Database.
Research topics: The Childcare Database comprises individually linked Danish register-based data on childcare attendance,
childcare facility characteristics, child and family characteristics, and infectious disease hospitalisations. The database
includes about 1 million children aged 0–5 years and has, since the creation, been linked with separate disease registers on
atopic disease, pneumococcal disease, and childhood cancers. The present paper is a review of epidemiological studies based
on the Childcare Database. Studies of childhood infections confirmed that childcare attendance dramatically increases the
risk, but emphasised that the increased risk is often transient and confined to subsets of children. Studies of childhood
cancers showed that early childhood infections are likely to reduce the risk of childhood leukaemia and that this risk reduction
applies to all children. Conclusion: The Childcare Database is a unique data source for studying the association
between childcare attendance and health outcomes. Further linkage with Danish registers is possible on an
individual level. The studies based on the Childcare Database confirm and extend previous findings of an
increased risk of infection associated with childcare attendance, as well as point towards a possible protective
role of early infections in childhood cancer.