This study investigated the epidemiology and determinants of off-label antibiotic use in children in Taiwan. This was a retrospective, population-based study using the 2002-2005 Taiwan's National Health Insurance Research Database (NHIRD). The study subjects consisted of children under 12 years old. The prescriptions of antibiotics were retrieved from the pharmacy claims of NHIRD. Multivariable logistie regression models were used to assess the associations between patients' and providers' demographics and off-label use of antibiotics. Among 66,446 antibiotic prescriptions identified, off-label use accounted for 33.33%. The percentages of off-label use varied, with time trend, dispensing type. patient characteristics, physician characteristics and institution characterìstics, Off- label use of antibiotics is very common among Taiwanese children. especially in the cases whose prescriptions were issued by senior and experienced physicians as well as those from private, non-teaching or regional hospitals. Off-label use can seriously affect pediatric drug safety and health, and are worthy of more concern and discussion. In addition, the study in the future would still need to explore the possible root causes of off label use.