Purpose: Medication-related adverse events (MRAEs) are an important priority for patient safety. Results from Dutch AE studies showed that—despite various improvement initiatives—the incidence of preventable MRAEs did not decline. The aim of this study is to describe the characteristics of MRAEs during hospitalizations using national patient data from records of patients admitted to Dutch hospitals in 2008 and 2011/2012. Methods: Trained nurses and physicians reviewed the randomly selected records of 8071 patients admitted to one of 20 hospitals in 2008 or 2011/2012 during a two-stage review process. Patient and admission characteristics were collected. After identification of a MRAE, physicians determined their potential preventability, drug type, related prescribing factors, and potential consequences. Results: The physicians identified 928 adverse events (AEs) in 857 admissions, of which 218 (15.2%) were medication-related. They judged 55 (18.4%) of these as preventable. Preventability of MRAEs was high in anticoagulant treatment (42.5%). Haematoma (39.0%) and intra-cerebral haemorrhage (25.5%) were common types of anticoagulant-related AEs. Anticoagulant-related AEs were often related to dosage factors (46.9%) and often resulted in an intervention (80.2%), of which 40.2% was judged as preventable. Conclusions: This study provided detailed information on MRAEs during hospital admissions in The Netherlands. A substantial proportion of AEs was medication-related (15.2%), of which 18.4% was judged to be preventable. As preventability in MRAEs was especially high in anticoagulant treatment (42.5%), those medications are a threat to patient safety. Future research and new safety programs should focus on prevention of AEs related to this medication group.