Objective: The objectives of this study are to determine the prevalence and preventability of adverse events requiring an unplanned higher level of care, defined as an unplanned transfer to the ICU or an in-hospital medical emergency team intervention, and to assess the type and the level of harm of each adverse event. Design: A three-stage retrospective review process of screening, record review, and consensus judgment was performed. Setting: Six Belgian acute hospitals. Patients: During a 6-month period, all patients with an unplanned need for a higher level of care were selected. Interventions: The records 6-month period, the records of all patients with an unplanned need for a higher level of care were assessed by a trained clinical team consisting of a research nurse, a physician, and a clinical pharmacist. Measurements and Main Results: Adverse events were found in 465 of the 830 reviewed patient records (56%). Of these, 215 (46%) were highly preventable. The overall incidence rate of patients being transferred to a higher level of care involving an adverse event was 117.6 (95% CI, 106.9-128.3) per 100,000 patient days at risk, of which 54.4 (95% CI, 47.15-61.65) per 100,000 patient days at risk involving a highly preventable adverse event. This means that 25.9% of all unplanned transfers to a higher level of care were associated with a highly preventable adverse event. The adverse events were mainly associated with drug therapy (25.6%), surgery (23.7%), diagnosis (12.4%), and system issues (12.4%). The level of harm varied from temporary harm (55.7%) to long-term or permanent impairment (19.1%) and death (25.2%). Although the direct causality is often hard to prove, it is reasonable to consider these adverse events as a contributing factor. Conclusion: Adverse events were found in 56% of the reviewed records, of which almost half were considered highly preventable. This means that one fourth of all unplanned transfers to a higher level of care were associated with a highly preventable adverse event. ...