This systematic review examines the incidence, preventability, and outcomes of in-hospital adverse events. The study included eight studies with a total of 74,485 patient records, yielding a median overall incidence of 9.2% and a median percentage of preventability of 43.5%. More than half of the patients experienced no or minor disability, while 7.4% of events were lethal. Operation- and medication-related events were the most common, accounting for 39.6% and 15.1% of cases, respectively. The review also highlights evidence-based interventions aimed at reducing these types of events, such as perioperative beta-blockade and antibiotic prophylaxis. Despite methodological limitations, the findings suggest that adverse events during hospital admission affect nearly one out of 10 patients, with a substantial portion being preventable.This systematic review examines the incidence, preventability, and outcomes of in-hospital adverse events. The study included eight studies with a total of 74,485 patient records, yielding a median overall incidence of 9.2% and a median percentage of preventability of 43.5%. More than half of the patients experienced no or minor disability, while 7.4% of events were lethal. Operation- and medication-related events were the most common, accounting for 39.6% and 15.1% of cases, respectively. The review also highlights evidence-based interventions aimed at reducing these types of events, such as perioperative beta-blockade and antibiotic prophylaxis. Despite methodological limitations, the findings suggest that adverse events during hospital admission affect nearly one out of 10 patients, with a substantial portion being preventable.