Background: The need to use unlicensed and off label drugs in children due to the lack of suitable, licensed formulations with appropriate prescribing information creates many problems on an everyday basis for healthcare professionals, carers and children. Patients and methods: This prospective study was designed to examine the incidence and nature of unlicensed and off label prescribing, in paediatric oncology patients with acute lymphoblastic leukaemia and other malignancies. Inpatient and outpatient prescriptions were analysed for a 4-week period. Results: All patients received at least one unlicensed or off label drug. Fifty-five per cent of prescriptions were licensed, 19% were unlicensed and 26% were licensed drugs used in an off label manner. Unlicensed preparations were used in 40% of prescriptions for cytotoxic agents, due to a lack of commercially available formulations suitable for the paediatric patient. These drugs included mercaptopurine and methotrexate which have been used in the treatment of paediatric leukaemia for many years, their efficacy having been demonstrated by on-going Medical Research Council trials. Conclusions: It is disappointing that drugs, which are the mainstay of therapy for paediatric leukaemia and other malignancies, are unavailable in appropriate licensed formulations to facilitate their administration to children. This needs to be urgently addressed.