OBJECTIVE: To study the effect of environmental tobacco smoke to children's health and more specifically to the development of respiratory tract infections and also its influence to their respiratory function. METHODS: We studied 586 children from the district of west Thrace in Greece aged 5-14 years, randomly selected from 20 primary and 10 secondary schools in regards their exposure to tobacco smoke, and its possible relation with respiratory tract infections. We assessed respiratory function by spirometry. Epidemiological data were also analysed. RESULTS: Children who were exposed to environmental tobacco smoke had higher risk for upper and lower respiratory tract infections in comparison to children who lived in a smoke free environment (3 fold and 2 fold, respectively. Their lung function was also reduced with decreased values of FEV1, FEF50, PFR (values p<0.001). CONCLUSION: Passive smoking has an unfavorable effect in respiratory function of children and predisposes them to respiratory tract infections. Every effort should be undertaken in order to avoid exposure of children to environmental tobacco smoke.