To examine the association between passive smoking and snoring in preschool children using parent-reported questionnaires and urine cotinine levels. This was a population-based cross-sectional survey of 2954 children aged 2-6 years in Hong Kong. Parent-reported questionnaires provided information on snoring and household smoking. One-third of children randomly chosen from the cohort provided urine samples for cotinine analysis. Increased urine cotinine was defined as urinary cotinine concentration ≥ 30 ng/mg creatinine. Using multivariate logistic regression analysis, we analyzed the association between snoring and passive smoking, controlling for potential confounders including age, sex, body mass index z-score, atopic diseases, recent upper respiratory tract infection, parental allergy, parental education, family income, and bedroom-sharing. A total of 2187 completed questionnaires were included in the final analysis, and 724 children provided urine samples for cotinine measurement. After adjustment for confounding factors, questionnaire-based household smoking (>10 cigarettes/d: OR = 2.22, 95% CI = 1.02-4.81) and increased urine cotinine (OR = 4.37, 95% CI = 1.13-16.95) were significant risk factors for habitual snoring (snoring ≥ 3 nights per week). For occasional snoring (snoring 1-2 nights per week), reported household smoking (1-10 cigarettes/d: OR = 1.41, 95% CI = 1.14-1.76; >10 cigarettes/d: OR = 1.56, 95% CI = 1.05-2.31), and increased urine cotinine (OR = 1.82, 95% CI = 1.03-3.20) were also identified as significant risk factors. A dose-effect relationship was found for snoring frequency and adjusted natural logarithms of urinary cotinine concentrations (P < .001). Environmental tobacco smoke exposure is an independent risk factor for snoring in preschool children. Parents' smoking cessation should be encouraged in management of childhood snoring.