In the last two decades, tremendous advances have been made in the treatment of acute lymphocytic leukaemia (ALL) in children with 5 year 'cure' rates in excess of 90%. The maintenance of remission is due, in part, to individualisation of therapy which must consider age, body size, genetic constitution and the impact of disease on drug disposition and action. This review, focused on treatment of ALL and one of the therapeutic mainstays, 6-mercaptopurine, illustrates the importance of obesity as a modulating factor in dose individualisation. Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://group.bmj.com/group/rights-licensing/permissions.