Background: Diagnosis and treatment for adult-onset AD is challenging. Many attempts have been made to characterize adult-onset AD in order to improve diagnosis, treatment and estimate clinical prognosis. So far, there have been few studies comparing clinical characteristics and laboratory findings in adult-onset and pre-adult-onset AD. Objectives: The aim of this study is to find out characteristics of adult-onset AD by comparing clinical characteristics and laboratory findings between adult-onset and pre-adult-onset AD. Methods: We classified AD patients into two groups with onset before and after 18 years of age. We performed analysis of EASI-score, total immunoglobulin E (IgE), eosinophil ratio, eosinophilic cationic protein (ECP), vitamin D, clinical phenotypes and affected areas. The results were compared between two groups. Results: Among total of 212 AD patients, 66 patients were included in adult-onset group and 146 patients in pre-adult-onset group. Pre-adult onset group with persistent AD revealed higher tendency of extrinsic phenotype and severer manifestations, showing higher total IgE and eosinophil ratio, lower vitamin D value. Patients with pre-adult onset showed more extensively affected area such as face, neck, scalp, ear, eyelid, lip, and flexural area especially ankle and wrist. Conclusion: These data suggest that there may be difference of laboratory findings and clinical characteristics between adult-onset and pre-adult-onset AD, more large-scale study is required.