Purpose: Atopic dermatitis may impair quality of life and lead to attention deficit-hyperactivity disorder (ADHD). Thymus and activation-regulated chemokine (TARC)/CCL17 may serve as a new biomarker for atopic dermatitis. We investigated the relationship between TARC and the severity of atopic dermatitis, quality of life, and ADHD. Methods: A total of 249 preschool children who had atopic dermatitis were enrolled. Parents of the patients filled out a questionnaire on the past history of allergic diseases, quality of life, and ADHD. In each patient, total immunoglobulin (Ig) E and specific IgE to nine foods and inhalant allergens, total eosinophil counts, and TARC levels were measured. We evaluated the severity of atopic dermatitis by using the scoring atopic dermatitis (SCORAD) score and divided the patients into three groups; mild (<15), moderate (15 to 40), and severe (>40). Results: In a total of 249 children, 222 children (89.2%) had a history of atopic dermatitis. Children with allergic sensitization had a higher SCORAD score, total IgE levels, and total eosinophil counts, but not TARC levels. Three groups by the SCORAD score showed significant differences in quality of life index and TARC levels but not in ADHD index. TARC level was correlated with the SCORAD score, but not with the quality of life index and ADHD index. The SCORAD score was correlated with the quality of life index but not with the ADHD index. Conclusion: Serum TARC levels may be associated with the severity of atopic dermatitis but not with the degree of quality of life and ADHD. Disease severity of atopic dermatitis in preschool children may be associated with the degree of quality of life but not with the level of ADHD. [Pediatr Allergy Respir Dis(Korea) 2011;21:215-225]