Interleukin (IL)-16 serves as a natural ligand of CD4 molecules and induces chemotaxis in CD4-expressing cells such as T cells, eosinophils, dendritic cells and monocytes. We examined the serum levels of IL-16 in patients with adult atopic dermatitis when their eruptions were aggravated and in non-atopic healthy controls, and then analysed the possible correlation between these values and the levels of several clinical markers. The serum levels of IL-16 were significantly higher in patients with atopic dermatitis than in the controls--both in exacerbation status and after conventional treatment. Multiple regression analyses showed that serum IL-16 was a predictor of the eosinophil count. Circulating IL-16 levels decreased significantly in patients with atopic dermatitis after topical treatment with corticosteroids or tacrolimus. These findings provide evidence that IL-16 plays a role in the exacerbation of chronic adult atopic dermatitis.