The objective of this study was to evaluate the effects of exposure to nonmaternal, passive household cigarette smoke on the incidence of respiratory illness (bronchitis and pneumonia) among children from birth through age 18 months in the Lu-Wan District, Shanghai City, People's Republic of China. A secondary objective was to evaluate the effects of other environmental factors, such as the fuel used for cooking and whether the child was breast fed, on the incidence of respiratory illness. The total daily cigarette consumption of family members was used to estimate exposure to passive smoke. The relative risks of exposure to passive cigarette smoke on the incidence of respiratory illness were 1.3, 1.7, and 2.0 for children living in households with members who smoked 1 to 9, 10 to 19, and 20 to 39 cigarettes per day, respectively, compared with the risks for children living in nonsmoking households (p for trend = 0.0002). These effects did not change materially when potential confounding factors were controlled. Children who were not fed human milk had a 1.8-fold increased risk of respiratory disease at each level of exposure to passive cigarette smoke evaluated, in comparison with children who were fed human milk for at least 1 month.