Objective: To determine the extent of use in children in hospital of drugs that are not specifically licensed for use in children (unlicensed) and of drugs that are used outside the terms of their product licence that apply to indication, age, dose, or route of administration (off label). Design: Prospective study of drugs administered on paediatric medical and surgical wards for 13 weeks. Setting: Regional children's hospital. Subjects: Paediatric inpatients in medical and surgical wards. Main outcome measures:Comparison of the use of each drug with its product licence to determine whether the drug was used in an unlicensed or off label manner. Results: 2013 courses of drugs were administered to 609 paediatric patients in 707 admissions. 506 (25%) of the drug courses (prescriptions) were either unlicensed (139) or off label (367) uses. In 256 (36%) of the 707 admissions patients received one or more courses of an unlicensed or off label treatment in hospital. Conclusions: Use of drugs in an off label or unlicensed manner to treat children is widespread. Drugs are more likely to be used in an off label manner than in an unlicensed manner.