Small effects of environmental tobacco smoke exposure on lung function have been demonstrated in many studies of children, but fewer studies have examined adults in this respect. We examined these relations in a 7-year longitudinal study of 1,623 British adults, age 18–73 years, who were nonsmokers throughout. Outcome was measured by forced expiratory volume in 1 second (FEV1) adjusted for sex, age, and height. Exposure was assessed by asking subjects whether they lived with a smoker (at both the initial and the follow-up studies) and by salivary cotinine measurements (follow-up study only). Cross-sectionally, subjects exposed at home showed tiny FEV1deficits at both studies of −4 ml [95% confidence limits (CL) = −31, 23] and −5 ml (95% CL = −32, 22), respectively. Cotinine adjusted for potential confounders showed a stronger association with FEV1, with the highest quintile showing a −105-ml deficit (95% CL = −174, −37) in comparison with the lowest. Longitudinally, no clear relation was apparent between change in FEV1and average exposure or change in exposure. These results indicate that environmental tobacco smoke is associated with small deficits in adult lung function, consistent with our meta-analysis estimate of a 2.7% deficit in exposed nonsmoking adults. The relations seen with cotinine but not with household exposure may reflect the importance of exposure outside the home. (Epidemiology 1999;10:319–326)