Elevated serum levels of neurotrophins such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) have been reported in allergic and autoimmune diseases. The aim of this study was to assess serum levels of BDNF in patients with atopic dermatitis (AD) and to investigate the relationship of the BDNF level with other markers of disease severity. Serum BDNF concentration was significantly higher in patients with AD (n=62) compared to control subjects (n=20) (P<0.01). Stepwise multiple regression analysis showed a significant influence of the peripheral blood eosinophil counts (F=6.90) and the percentage of CD4(+)IL-4(+) (Th2) cells (F=6.61). Moreover, after remission of AD patients with conventionally treated AD patients (n=14), serum levels of BDNF, eosinophil counts and percentage of Th2 cells were decreased significantly. These results suggest that serum BDNF may be a useful marker of disease activity in AD and that both eosinophils and Th2 cells are major cellular sources of serum BDNF.