The effects of exposure to environmental tobacco smoke (ETS) on pulmonary function were investigated in a random sample of 1,033 adults aged 40 to 69 yr from a residential area in Beijing. Compared with those not exposed, those exposed to ETS either at home or at work were associated with a 102-ml (SE = 40 ml) reduction in FEV1 and a 151-ml (SE = 44 ml) reduction in FVC after adjustment for confounding factors. Home and work ETS exposure was respectively associated with a 102-ml (SE = 33 ml) and 61-ml (SE = 34 ml) reduction in FEV1. When the subjects were grouped into three exposure categories (none, home only or work only, and both), the deficits in FEV1 and FVC in the third category were the greatest and statistically significant. The adverse effects of ETS were consistently observed in stratified analyses according to sex, occupational exposure, indoor use of coal stoves, and education. When ETS exposure was measured by the cigarettes smoked by other household members at home per day, an exposure-response association with FEV1 and FVC was again statistically significant. In conclusion, this study demonstrated that there is a significant association between exposure to ETS and reduced levels of FEV1 and FVC in adults, and such an association is dose-dependent.