Eosinophil cationic protein (ECP) is a cationic protein derived from eosinophil granulocytes, and has been studied mainly in atopic diseases and considered as a useful marker of disease activity in atopic dermatitis. We measured the serum ECP levels in patients with various skin diseases (n = 875) and in normal healthy controls (n = 79), and evaluated the correlation between ECP level and blood eosinophil number, or ECP and IgE levels. Serum ECP levels were significantly higher in patients with drug eruption (15.8 +/- 1.7 micrograms/l), psoriasis (15.1 +/- 6.0 micrograms/l), acute urticaria (13.9 +/- 1.4 micrograms/l) than in healthy controls (4.5 +/- 0.3 micrograms/l) (P < 0.05) and also significantly elevated in patients with elevated eosinophil numbers (15.2 +/- 1.0 micrograms/l) compared to those in patients with normal eosinophil numbers (8.8 +/- 0.3 micrograms/l) (P < 0.001). Serum ECP level and eosinophil number in peripheral blood were also correlated in patients with psoriasis (gamma = 0.82, P < 0.01), drug eruption (gamma = 0.31, P < 0.01) and acute urticaria (gamma = 0.20, P < 0.05). However, no correlation between ECP and IgE levels in all of the patients was found. Among the patients with chronic urticaria, ECP levels showed an increasing trend in patients with angioedema, cold urticaria and dermographic urticaria as compared with those in healthy controls. Our results suggest that, even though the role of ECP released from activated eosinophils is still unknown, its measurement might be of help to understand the pathogenesis of some skin disorders.