OBJECTIVE: To assess the validity of using International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) code 710.3 to identify adult patients with dermatomyositis in outpatient and inpatient settings. METHODS: Electronic medical records of adult patients with ICD-9 code 710.3 between January 2001 and November 2014 (n = 511) were examined. Physician diagnosis, clinical findings, and diagnostic testing results were recorded. A dermatomyositis rating scale was assigned based on classic cutaneous findings and at least 2 additional clinical and diagnostic findings from the Bohan criteria. Sensitivity and positive predictive values (PPVs) were determined. Sensitivity analyses were performed to evaluate the accuracy of multiple ICD-9 codes in the outpatient setting, as well as primary and secondary inpatient codes. RESULTS: The sensitivity and PPV for multiple 710.3 ICD-9 codes in the outpatient setting were 0.89 and 0.35, respectively. The PPV for primary and secondary 710.3 inpatient codes was 0.95 and as high as 0.8. However, the sensitivity of ICD-9 code 710.3 was poor in the inpatient setting (primary 0.23 and secondary 0.26). The most common reason for failure to meet appropriate dermatomyositis criteria was miscoding as diabetes mellitus (32%), followed by diagnosis at an outside institution (19%), dermatomyositis as a rule-out diagnosis (10%), cutaneous dermatomyositis (8%), and juvenile dermatomyositis (6%). CONCLUSION: One or more occurrences of ICD-9 code 710.3 is insufficient to support the diagnosis of dermatomyositis in the outpatient setting. However, ICD-9 710.3 codes appear to be valid in the inpatient setting.