Environmental tobacco smoke is an important public health problem. The objective of this study was to evaluate the effect of passive smoking on lower respiratory tract infections (LRTIs) in children aged 2 - 12 years. A case-control study was conducted on matched-pair design. One-hundred and fifty children with LRTIs and 150 healthy children were included in the study. Data were collected through questionnaire and urine samples for the determination of cotinine levels, and were analysed by McNemar chi-square, paired t-test and Pearson correlation tests. The prevalence of parental self-reported, indoor smoking was 71.3% in children with LRTI and 72.0% in healthy children. Employing 30 ng mg(-1), the cut-off level of urinary cotinine/creatinine as commonly accepted, 87.3% of the children with LRTIs and 84.7% of healthy children were found to be passive smokers ( p = 0.61, odds ratio (OR) 0.93; confidence interval (CI) = 0.34 - 2.53). If 60 ng mg (-1) of urinary cotinine/creatinine was accepted as a cut-off level, it was observed that the rates of passive smoking were 76.7% and 50.7%, respectively (p = 0.000, OR = 4.72; 95% CI = 2.62 - 8.52). Dose-dependent exposure to environmental tobacco smoke was found to be associated with the incidence of LRTI.