Dr Angela Thomas is an honorary senior lecturer within Child Life and Health at the University of Edinburgh. From First World War observations that mustard gas caused lymphoid tissue to dissolve and that folate seemed to accelerate the leukaemic process, carefully chosen chemicals were used to tackle leukaemia, sometimes with startling success. In the early days, some children were cured with this minimal therapy. As more drugs were discovered, so treatment intensified and more children were cured. The current challenge is to identify which children will do well with less treatment and who need more. This is where the new DNA technologies have proved so helpful to ensure that children with leukaemia have the maximum chance of cure with the minimum side effects. This lecture is part of the Medical Detectives, a series of public lectures that show how keen detective work is still essential for 21st century doctors. Recorded on 26 September 2013 in the Anatomy Lecture Theatre at the University of Edinburgh.