The evidence of an association between passive smoking and occurrence of respiratory symptoms is relatively strong in children, whereas studies conducted in adult populations have provided inconsistent results. The objective of the present study was to examine the relations between exposure to environmental tobacco smoke (ETS) and development of respiratory symptoms in young adults during a study period of 8 years, with emphasis on the evaluation of potential dose-response pattern of the relations. The study population consisted of 117 "never smokers," who were 15 to 40 years of age at the time of initial examination, when they answered a standardized questionnaire on respiratory health, and who were reexamined 8 years later. ETS exposure at home and at work during the study period was recorded at the 8-year examination with a structured questionnaire. The symptoms studied as outcomes included wheezing, dyspnea, cough, and phlegm production. The relations between ETS exposure and development of respiratory symptoms were studied in multivariate logistic regression models controlling for age, gender, atopy, and the presence of other respiratory symptoms. Cumulative incidences of the respiratory symptoms, except of phlegm production, were consistently greater among subjects exposed to ETS compared with the reference group. A significant dose-related increase in the risk of developing dyspnea was observed in relation to ETS exposure, with an OR of 2.37 for an average exposure of 10 cigarettes/day (95% confidence interval, 1.25-4.51). The risk of developing other respiratory symptoms, apart from phlegm, was also related to ETS exposure, but these relations did not achieve statistical significance. The results provide evidence of adverse respiratory effects of ETS exposure in the home and office work environments in young adults. These findings emphasize the need for effective measures in the prevention of involuntary smoking during young adulthood.