Adverse events (AEs) occur in health care and may result in harm to patients especially in the field of surgery. Our objective was to analyze AEs in surgical patient care from a nationwide perspective and to analyze the frequency of AEs that may be preventable. In total 19,141 randomly selected admissions in 63 Swedish hospitals were reviewed each month during 2013 using a 2-stage record review method based on the identification of predefined triggers. The subgroup of 3301 surgical admissions was analyzed. All AEs were categorized according to site, type, level of severity, and degree of preventability. We reviewed 3301 patients' records and 507 (15.4%) were associated with AEs. A total of 62.5% of the AEs were considered probably preventable, over half contributed to prolonged hospital care or readmission, and 4.7% to permanent harm or death. Healthcare acquired infections composed of more than one third of AEs. The majority of the most serious AEs composed of healthcare acquired infections and surgical or other invasive AEs. The incidence of AEs was 13% in patients 18 to 64 years old and 17% in ≥65 years. Pressure sores and drug-related AEs were more common in patients being ≥65 years. Urinary retention and pressure sores showed the highest degree of preventability. Patients with probably preventable AEs had in median 7.1 days longer hospital stay. We conclude that AEs are common in surgical care and the majority are probably preventable.