Eosinophil cationic protein (ECP) levels may be of value in assessing tissue inflammation in atopic diseases. However, conflicting results have been reported regarding levels of serum ECP (s-ECP) and symptom activity in asthma, atopic dermatitis, or allergic rhinitis. To examine s-ECP levels, eosinophil counts and total immunoglobulin E (IgE) in relation to atopic disease activity in a random population of children admitted to hospital for atopic and nonatopic conditions. s-ECP, eosinophil counts, and total IgE were evaluated in all the children admitted, to a pediatric ward, excluding patients treated with systemic steroids. A total of 216 children were studied. Shin prick tests (SPT) and a validated questionnaire for asthma, atopic: dermatitis, and allergic rhinitis were obtained. s-ECP was significantly higher in children with symptomatic atopic disease (19.88 mu g/L) compared with nonatopic control patients (8.58 mu g/L). The levels of s-ECP were also elevated in patients with a positive response to the ISAAC Questionnaire for symptoms of asthma, atopic dermatitis, or rhinitis in the previous 12 months (9.12 mu g/L) or a positive SPT to relevant allergens (8.58 mu g/L), but no symptoms of atopy at the time of examination. Intermediate values of s-ECP were found in children with combined positive SPT and positive response to the ISAAC Questionnaire (13.7 mu g/L), but no symptoms of atopic disease at the time of examination. s-ECP is more elevated in symptomatic atopic subjects than in nonallergic patients. Subjects with latent (nonsymptomatic) allergy conditions show intermediate values probably reflecting allergen exposure and minimal persistent tissue inflammation.