Eosinophil cationic protein (ECP), one of the eosinophil granule proteins, is released during allergic reactions. We investigated the possibility of correlations among the serum levels of ECP, clinical activity, and eosinophil number in patients with atopic dermatitis (AD). Forty-four patients with AD and 25 normal, non-atopic subjects were studied. ECP was quantitated by a double antibody radioimmunoassay. The levels of serum ECP correlate with the grading of severity of clinical evaluations in AD. The patients with severe and moderate AD had significantly higher ECP concentrations than normal controls (p less than 0.001); mild AD had levels identical with those of control groups. A positive correlation was observed between the number of peripheral blood eosinophils and serum ECP levels in the severe cases (r = 0.67, p less than 0.05). Furthermore, these ECP levels significantly decreased in response to either improvement of clinical severity of AD or decreased numbers of blood hypodense eosinophils in anti-allergic drug-treated patients. No coefficient of correlation was observed between serum ECP and IgE levels. These findings indicate that eosinophils may release their granular contents, including ECP, into the peripheral circulation and/or inflammatory skin lesions and subsequently provoke a clinical exacerbation by stimulating allergic reactions.