Background: Recently, nationwide medical research of psoriasis using National Health Insurance Service database is increasing. However, identification of psoriasis using diagnostic codes alone can lead to misclassification. Accuracy of the diagnostic codes and concordance with electronic medical records should be validated to identify psoriasis patient Objectives: To validate the diagnostic codes of psoriasis (ICD-10 L40) and to find algorithm for identification of psoriasis Methods: We collected medical records of patients who got their first diagnostic codes of psoriasis during 5 years (2012-2016) from five hospitals. 15 percent of patients were randomly selected at each hospital. Patients were classified as psoriasis, not psoriasis, and questionable for final diagnosis. We validated several algorithms to identify psoriasis Results: Total 538 cases were reviewed and classified as psoriasis (n=368), not psoriasis (n=159), and questionable (n=11). The most accurate algorithm was including patients with primary diagnostic codes of psoriasis and prescription of vitamin D derivatives. Positive predictive value was 96.5% (95% CI, 93.9~98.1%) which were significantly higher than including patients with psoriasis diagnostic codes alone (p=0.002, 96.5% vs. 91.0%). Sensitivity was 90.8% (95% CI, 87.2~93.4%) and specificity was 92.4% (95% CI, 86.9~95.9%) Conclusion: Our study demonstrates validated algorithm to identify psoriasis which is useful for the nationwide population based study for psoriasis