Patients with atopic dermatitis frequently show elevated blood eosinophil counts, and eosinophil-derived major basic protein has been demonstrated in the eczematous skin from patients with atopic dermatitis. To evaluate further the role of eosinophils in the pathogenesis of atopic dermatitis, the concentration of eosinophil cationic protein was measured in serum samples of 42 patients with moderate to severe disease. The results were compared with those obtained in 32 patients with psoriasis with (n = 9) or without (n = 23) a history of inhalant allergy, 12 patients with a history of pseudoallergic reactions to acetylsalicylic acid, 14 patients with a history of inhalant allergy, and 31 nonatopic healthy control subjects. Eosinophil cationic protein levels were significantly increased in the serum of patients with atopic dermatitis (p less than or equal to 0.005) and patients with a history of pseudoallergic reactions to acetylsalicylic acid (p less than or equal to 0.01). There was no significant difference between eosinophil cationic protein levels in patients with psoriasis or a history of inhalant allergy and in control subjects. Moreover, eosinophil cationic protein levels did not differ significantly in psoriasis patients with or without inhalant allergy. These studies support the concept of an active participation of eosinophils in atopic dermatitis and point to a possible role for eosinophils in pseudoallergy.