BACKGROUND: It has been suggested that childhood asthma lowers the risk of childhood leukaemia. Studies have found an inverse association between these conditions. However, most studies on this relationship are based on questionnaires and telephone interviews, introducing recall bias. Therefore, we conducted a matched case-control study based on drug prescription data to assess the relationship between both conditions. METHODS: In a large database, covering more than one million individuals, we identified cases of children who had been prescribed 6-mercaptopurine (6-MP). This drug is used in the outpatient maintenance therapy of childhood leukaemia. We matched every child with leukaemia on sex and age (±6 months) to children without leukaemia (controls). The variable of having had asthma was defined as receiving at least two prescriptions for an inhaled corticosteroid within 12 months. RESULTS: We identified 59 children aged 2-18 who had been prescribed 6-MP (cases), and they were matched to 21,918 controls. Of the children with childhood leukaemia, three (5%) had childhood asthma, whereas in the control group 4889 (22%) had childhood asthma (odds ratio [OR] 0.19; 95% confidence interval 0.06-0.60). CONCLUSION: In this study on the relationship between childhood asthma and childhood leukaemia, we found a strong inverse association.