The role of eosinophil cationic protein (ECP) in allergic inflammatory reaction has been documented in experimental and clinical studies, but a clinical usefulness is still discussed. In the study serum level of ECP has been evaluated in children with allergic diseases of the respiratory system in exacerbation and remission of symptoms for purpose of monitoring of disease course. In 111 children aged 12.0 +/- 3.3 yrs with atopic bronchial asthma and/ or allergic rhinits ECP serum concentrations have been determined in following groups: children with grass pollen hypersensitivity (group P, 17 female and 41 male), children with hypersensitivity to D. pteronyssinus and D. farinae (group D, 16 female and 37 male) and controls without allergic hypersensitivity with negative prick skin tests (19 children, 11 female and 5 male). All children have been qualified to immunotherapy with pollen or mite allergens (Allergovit or Novo-Helisen, Nexter, Allergopharma) and ECP evaluation was performed before, during and after therapy. Serum ECP and IgE levels have been determined with CAP-system (Pharmacia) and obtained results related to clinical symptoms. In all analyzed children serum total IgE has been significantly increased in relation to controls. Serum ECP levels have been increased during clinical exacerbation of symptoms in observed children and parallel with clinical score of symptoms, especially during pollen season. Authors conclude that a degree of increase of serum ECP level is parallel with clinical score of symptoms, especially during highest exposition to pollen allergens. Observed changes of serum ECP levels during immunotherapy suggest the close relationship with allergic inflammatory reaction and indicate clinical usefulness for monitoring of this process.