Short-term exposure to air pollution has been associated with exacerbation of chronic obstructive pulmonary disease (COPD), whereas the role of long-term exposures on the development of COPD is not yet fully understood. We assessed the effect of exposure to traffic-related air pollution over 35 years on the incidence of COPD in a prospective cohort study. We followed 57,053 participants in the Danish Diet, Cancer, and Health cohort in the Hospital Discharge Register for their first hospital admission for COPD between 1993 and 2006. We estimated the annual mean levels of nitrogen dioxide (NO₂) and nitrogen oxides (NO(x)) at all residential addresses of the cohort participants since 1971 to an event or 2006 and used indicators of traffic near the residential address at recruitment. We assessed the association between exposure to air pollution and COPD incidence by Cox regression analyses for the full cohort, and for participants with and without comorbid conditions, including asthma, diabetes, or cardiovascular disease. A first hospital admission for COPD was recorded for 1,786 (3.4%) of 52,799 eligible subjects between recruitment (1993-1997) and 2006. COPD incidence was associated with the 35-year mean NO₂ level (hazard ratio, 1.08; 95% confidence interval, 1.02-1.14, per interquartile range of 5.8 μg/m³), with stronger associations in subjects with diabetes (1.29; 1.05-1.50) and asthma (1.19; 1.03-1.38). Long-term exposure to traffic-related air pollution may contribute to the development of COPD with possibly enhanced susceptibility in people with diabetes and asthma.