BACKGROUND: ICD codes are used to identify patients with appendicitis and to classify disease severity for reimbursement and research purposes. We sought to compare the accuracy of ICD-9 vs ICD-10 codes in classifying appendicitis as uncomplicated vs complicated (defined as perforated, necrotic, or abscess) compared with the clinical gold standard: surgeon characterization of the appendix in the operative report. STUDY DESIGN: This is a retrospective review of operative reports and discharge ICD-9/10 codes for patients 18 years or older who underwent noninterval, nonincidental appendectomy from January 2012 to December 2019 at a tertiary referral center. Sensitivity, specificity, and positive predictive value were calculated for ICD-9/10 codes to classify appendicitis as complicated when compared with surgeon description. Chi-square testing was used to compare agreement between ICD-9/10 codes and surgeon description. RESULTS: A total of 1,585 patients underwent appendectomy. ICD-9 codes had higher sensitivity than ICD-10 codes for complicated appendicitis (sensitivity 0.84 and 0.54, respectively) and a similar positive predictive value (0.77 and 0.76, respectively). Overall, 91% of ICD-9 codes agreed with surgical description of disease, but 84.4% of ICD-10 codes agreed with surgical description (p < 0.01). Among cases classified as complicated by the surgeon, 84% (79/94) had an accurate ICD-9 code for complicated disease, but only 53.8% (57/106) of cases had an accurate ICD-10 code (p < 0.01). CONCLUSIONS: Compared with ICD-9 codes, ICD-10 codes were less accurate in characterizing severity of appendicitis. The ICD-10 coding schema does not provide an accurate representation of disease severity. Until this system is improved, significant caution is needed for people who rely on these data for billing, quality improvement, and research purposes.