Rationale: Individuals exposed to second-hand smoking may be more susceptible to asthma and chronic obstructive pulmonary disease (COPD). We investigated the risk of respiratory symptoms, asthma, and COPD in adults exposed to second-hand smoking at different stages of life in the general population. Methods: We identified individuals who had been exposed to second-hand smoking in childhood only, adulthood only, or lifelong in a cohort of 20,421 adults from the Danish General Suburban Population Study and recorded respiratory symptoms, lung function, asthma, and COPD as outcomes. Results: Among 20,421 adults from the general population, 2,551 (12%) had been lifelong exposed to second-hand smoking, 459 (2%) had been exposed in adulthood only, and 13,998 (69%) had been exposed in childhood only; the mean ages of the three groups were 54 years, 55 years, and 57 years, respectably, compared with 56 years in non-exposed individuals (P<0.001). Equivalent values for the prevalence of current smoking were 25%, 20%, and 18% versus 12% (P<0.001). After adjustment for age, smoking, and sex, the odds ratios for wheezing, severe dyspnoea, cough on exertion, and asthma increased as a function of second-hand smoke exposure (Ps≤0.004); individuals who had been exposed to second-hand smoking lifelong, in adulthood only, or in childhood only versus non-exposed had increased odds ratios for wheezing of 1.62 (95% CI=1.41–1.87), 1.50 (1.15–1.94), and 1.16 (1.04–1.30). Corresponding values were 2.08 (1.52–2.85), 2.05 (1.22-3-44), and 1.23 (0.95– 1.59) for severe dyspnoea, 1.56 (1.33–1.83), 1.53 (1.15–2.02), and 1.19 (1.05–1.35) for cough on exertion, 1.36 (1.14–1.63), 1.49 (1.09–2.05), and 1.13 (0.99–1.30) for asthma, and 1.24 (1.03–1.48), 1.25 (0.90–1.74), and 1.09 (0.96–1.24) for COPD. The population attributable fractions of asthma and COPD due to lifelong second-hand smoke exposure were 4.3% and 2.9%. Conclusion: Individuals exposed to lifelong second-hand smoking have increased risks of respiratory symptoms, asthma, and COPD, and may account for 4.3% and 2.9% of people with asthma and COPD in the general population.