Twenty-two children (13 boys and 9 girls) with chronic cough were treated with the leukotriene receptor antagonist montelukast (Singulair tbl. 5 mg) administered once daily for four weeks. In 14 children (68%), the cough ceased during the third week of treatment. Children responding to montelukast were found to have higher blood levels of eosinophil cationic protein (S-ECP) in the pretreatment blood sample than children with no response (responders 14.88+/-2.651 microg/l versus nonresponders 6.62+/-0.948 microg/l; p<0.01). Blood S-ECP levels remained higher also in the post-treatment blood sample in responders (10.55+/-1.631 microg/l) compared to nonresponders (6.13+/-0.937 microg/l; p<0.05). The difference is statistically significant. There were also differences in absolute eosinophil blood count and IgE blood levels between the two groups in the pretreatment blood sample. Using 24-hour pH-metry, two children not responding to therapy were subsequently diagnosed to have gastroesophageal reflux. Judging from the results, one might deduct that patients with chronic cough who have increased levels of serum ECP and absolute eosinophil blood counts are likely to benefit from treatment with montelukast.