We conducted a population-based household survey of respiratory disease in 2,029 children and adults and measured salivary cotinine levels by radioimmunoassay in 1,360 nonsmokers and ex-smokers. At all ages median and mean cotinine levels among nonsmokers and ex-smokers increased with the number of smokers in the home. The prevalence of a detectable level of cotinine was about 35% for those not living with a cigarette smoker and was greater with the number of cigarettes smoked by household members. In a multiple logistic regression model, the major determinants of a detectable level of cotinine in children were mother's smoking (odds ratio (OR) = 3.2), father's smoking (OR = 2.1), and smoking of other household members (OR = 4.0). Among adults, the effects of spouse's smoking were smaller with OR = 1.3 and 1.4 for husband's and wife's smoking, respectively. We conclude that in the general population cotinine can be frequently detected in the saliva of nonsmokers, even among those not living with a smoker.