Although tobacco smoking is the major environmental risk factor for COPD, only a minority of smokers develops this condition, and it appears that genetic and other environmental factors are important in determining risk. There has been growing interest in the hypothesis that dietary factors modify COPD risk, possibly by protecting against oxidant injury. Available data addressing this hypothesis have a variety of methodological limitations that preclude any firm conclusions. The study was the first to address this hypothesis using data from the Nurses' Health Study, a large, prospective cohort study with detailed dietary assessments and a follow-up interval of sufficient duration to examine incident COPD.