Interleukin (IL)-18 is a pleiotropic cytokine that plays an important role in both type 1 (Th1) and type 2 (Th2) helper T lymphocyte-mediated immunity. Previous studies have suggested that IL-18 may be an inflammatory marker for atopic dermatitis (AD). The purpose of our study was to test whether the serum concentration of IL-18 is a useful inflammatory marker for assessing AD severity in young children. Nineteen AD patients with a median age of 2.2 years (interquartile range 0.7-4.6 years) were recruited. The severity of AD was clinically determined using the Scoring Atopic Dermatitis (SCORAD) index. Their SCORAD score was 23.9 (range 18.6-34.8). Serum IL-18 levels were determined by sandwich enzyme immunoassay. The median serum concentration of IL-18 was 394 pg/ml (interquartile range 204-612 pg/ml). Serum IL-18 levels correlated with SCORAD scores (r = 0.502, p = 0.029) and their extent component (r = 0.633, p = 0.004). When compared with mild disease with low SCORAD scores, the serum concentration in moderate to severe disease was significantly higher (p = 0.014). We concluded that serum IL-18 concentration is elevated in young children with AD. It may be a useful inflammatory marker that correlates with the extent component of AD in particular, and differentiates mild disease from more severe disease when used for assessing AD severity in young children.