Objectives: This study estimated the validity of 23 groups of codes in the International Classification of Diseases, 9th revision, Clinical Modification (ICD-9-CM or ICD), for flagging adverse events (AEs) related to hospital surgeries. Methods: A set of ICD codes selected as “flags” for in-hospital AEs were developed with the assistance of a national expert panel. The codes were grouped into 66 AE classes, 23 of which were identified as potentially surgery-related. The predictive value positive (PVP) of the 23 classes was assessed by medical record review of 941 surgical discharges. Results: The 23 classes identified AEs associated with care management during the hospital stay with an average PVP of 45 percent. They identified AEs specifically related to surgery with a PVP of 37 percent. Conclusions: This exploratory study identified a small number of classes and individual ICD codes that identified surgery-related AEs. Further study of selected codes is needed to refine the classes and fully evaluate their use for statewide surveillance of surgery AEs.