The study examined the relationship between dietary factors and chronic obstructive pulmonary disease (COPD) among participants in the Nurses' Health Study, an ongoing prospective cohort study of 121,700 women, ages 39-64 in 1985. This cohort had been followed by means of biennial questionnaires which inquired about a variety of topics, including dietary intake (using a validated semi-quantitative food frequency questionnaire) and physician diagnosis of COPD. In 1998, all participants with a history of COPD were sent a supplementary questionnaire regarding specifics of COPD diagnosis and related topics. The study examined the relation of dietary factors to risk of newly-diagnosed COPD during 1985-1998. During this time period, there were approximately 2,100 cases of "confirmed" COPD (i.e., physician diagnosis and pulmonary function tests [PFTs] at time of diagnosis or abnormal FEV-I in past year) and "probable" COPD (i.e., physician diagnosis and recent respiratory symptoms, but PFTs not known). Preliminary data supported the validity of these case definitions, and this was examined further by reviewing 600 medical records. Likewise, potential under-diagnosis was examined in a random sample of past and current smokers who had never reported COPD or asthma. The specific dietary hypotheses were that high intakes of antioxidants (e.g., vitamin C, vitamin E, and carotenoids), magnesium, potassium, and n-3 polyunsaturated fatty acids (e.g., fish oils) decreased risk of COPD, whereas high intakes of specific fatty acids (e.g., linoieic acid) increased risk. The cohort size and 13-year follow-up provided >90 percent power to detect a trend across quintiles of dietary intake. In 1998, among approximately 2,400 prevalent cases with diet data, study investigators addressed a secondary aim: to determine the relation of dietary factors to COPD severity during 1998-2000. COPD severity was assessed by self-report of current medications, recent symptoms, activity limitations, and health care utilization (e.g., emergency room or urgent office visits for COPD exacerbations). The rising prevalence of COPD, particularly among women, along with its high societal cost, makes COPD prevention an important public health goal.