Background: Atopic dermatitis (AD) may develop by 6 months of age, and its severity assessment is essential for appropriate treatments. Scoring Atopic Dermatitis (SCORAD) is suggested to evaluate the severity of AD but is cumbersome for routine clinical use. The serum thymus and activation-regulated chemokine (TARC) is used as a marker of AD severity. However, the normal range of the TARC levels varies by age, and its usefulness for the evaluation of AD severity has not been established in patients ages < 6 months. Here, we evaluated the correlation between serum TARC levels and SCORAD scores in early infancy and sought the optimal cutoff level to indicate AD severity. Methods: The subjects were 35 patients with AD (16 girls and 19 boys; 3-5 months of age) who visited our clinic between April 2015 and March 2017. All the patients were physically examined by a board-certified allergist. The AD severity was determined by using the SCORAD, together with serum levels of TARC, total immunoglobulin E (IgE), lactate dehydrogenase, and peripheral eosinophil counts. Receiver operating characteristic curve analysis was performed to determine the cutoff levels of serum TARC to indicate AD severity. Results: Significant correlations were observed between SCORAD scores and the serum TARC levels, peripheral eosinophil counts, and serum IgE levels (r = 0.640, r = 0.723, r = 0.533, respectively). The optimal cutoff levels of serum TARC to indicate mild and severe AD were <3523 pg/mL (area under the curve [AUC] = 0.856) and >6192 pg/mL (AUC = 0.833), respectively. Conclusion: Although this study had limitations, we suggest that serum TARC is useful as a marker of AD severity in patients <6 months of age.