Introduction:Studies show that tobacco smoke exposure adversely affects children?s lung function.Aim:The aim of this study was to determine the effect of secondhand tobacco smoke (STS) on lung functions in children with CF or PCD.Methods:Non-smoker PCD(n:53) or CF(n:61) patients aged 8-18 years were enrolled.Urine cotinine and serum inflammatory markers were measured with commercialized ELISA kits.Spirometry was performed at the same visit.Cotinine level was dichotomized as ?low? or ?high? with a cut-off of<12 ng/ml in patients based on previous studies.Multiple stepwise linear regression analysis was performed to adjust the effect of potential confounders on FEV1z score.Results:The median FEV1 z score was significantly lower in PCD patients compared to CF patients(-1.6(-3.0- -0.43)vs -0.2(-1.74-0.95 )(p<0.001).Serum IL4,IL6,IL 8,IL 17,MMP9,TIMP1 levels were similar between patients with low and high urinary cotinine levels, whereas median FEV1z score was significantly lower in patients with high urinary cotinine levels(FEV1:-0.7(-2.2-0.6 vs -1.3(-2.7-0.06)p:0.02).There was a significant low-grade negative correlation between urinary cotinine level and FEV1 z score(r:-0.22,p:0.02). The result of multivariate analysis showed that FEV1 zscore in CF patients was 1.39 unit higher than PCD patients and 1unit increase in urinary cotinine levels results in 0.14 unit decrease in FEV1z score.Conclusions:The results of this study show that high urinary cotinine level is associated with lower FEV1 z score.Future longitudinal studies with objective measures of cotinine levels are needed to clarify the relationship between secondhand TSE and lung function parameters in children with CF or PCD.