Background. Antibiotics are among the most commonly used drugs in a paediatric intensive care unit (PICU). Despite guidelines and protocols for the use of antibiotics, inappropriate use may contribute to an increase in antibiotic resistance. The factors behind changes in antibiotic prescriptions in the PICU at Grey's Hospital are unknown. Objective. To establish the frequency, process and rationale behind antibiotic prescription changes in the PICU. Methods. A retrospective descriptive study of all eligible patients admitted to the PICU during a 6-month period. Results. Three-quarters of patients admitted to the PICU received antibiotics during their stay. The antibiotic prescription was changed in 80 (58%) of the138 patients, with most changes (63.4%) occurring within 3 days of admission. Patients younger than 1 year and those who were malnourished accounted for 57% of the changes. The majority (65%) of the changes entailed the escalation of antibiotics and 89% of these were empiric therapy. De-escalation accounted for 35% of the changes. The rationale for a prescription change was not documented in 80% of cases. Conclusion. Antibiotic use in this PICU and changes to prescriptions were common. Changes were generally made on an empirical basis soon after admission and were more likely to occur in young malnourished children and patients admitted for a medical reason or surgical emergency.This open-access article is distributed under Creative Commons licence CC-BY-NC 4.0.