Objectives Knowledge about the epidemiology of adverse drug events (ADEs) and medication errors in paediatric inpatients is limited outside Western countries. To improve paediatric patient safety worldwide, assessing local epidemiology is essential.

Design The Japan Adverse Drug Events (JADE) Study was a cohort study.

Setting Paediatric inpatients at two tertiary care teaching hospitals in Japan.

Main outcome measures ADEs and medication errors identified by onsite review of all medical charts, incident reports and prescription queries by pharmacists. Two independent physicians reviewed all incidents and classified ADEs and medication errors, as well as their severity and preventability.

Results We enrolled 1189 admissions which included 12 691 patient-days during the study period, and identified 480 ADEs and 826 medication errors. The incidence of ADEs was 37.8 (95% CI 34.4 to 41.2) per 1000 patient-days and that of medication errors was 65.1 (95% CI 60.6 to 69.5) per 1000 patient-days. Among ADEs, 4%, 23% and 73% were fatal or life-threatening, serious and significant, respectively. Among the 480 ADEs, 36 (8%) were considered to be preventable which accounted for 4% of all medication errors, while 668 (81%) of all medication errors were judged to have the potential to cause harm to patients. The most common error stage for preventable ADEs was monitoring (78%) whereas 95% of potential ADEs occurred at the ordering stage.

Conclusions ADEs and medication errors were common in paediatric inpatients in Japan, though the proportion of ADEs that were preventable was low. The ordering and monitoring stages appeared most important for improving safety.