Leukaemia is the most common cancer in children. Although its aetiology remains unclear, recent evidence strongly suggests that a substantial fraction of childhood leukaemias originate in utero. Infection of the mother during pregnancy, transmitted to the foetus, is an important potential cause of genetic or immunological abnormalities and may lead to childhood leukaemia. Over quite a lengthy period various groups around the world have examined the association between maternal infection and risk of childhood leukaemia, but the results were inconsistent and mainly derived from case-control studies with relatively low-quality data. In this seminar, Jeff will summarise the existing evidence and then present some preliminary results using prospective data from the International Childhood Cancer Cohort Consortium (I4C).