Few studies have investigated the factors associated with the early inception of chronic obstructive pulmonary disease (COPD). We investigated COPD risk factors in an international cohort of young adults using different spirometric definitions of the disease. We studied 4,636 subjects without asthma who had prebronchodilator FEV(1)/FVC measured in the European Community Respiratory Health Survey both in 1991 to 1993 (when they were 20-44 yr old) and in 1999 to 2002. COPD was defined according to the Global Initiative for Chronic Obstructive Lung Disease fixed cut-off criterion (FEV(1)/FVC < 0.70), and two criteria based on the Quanjer and LuftiBus reference equations (FEV(1)/FVC less than lower limit of normal). COPD determinants were studied using two-level Poisson regression models. COPD incidence ranged from 1.85 (lower limit of normal [Quanjer]) to 2.88 (Global Initiative for Chronic Obstructive Lung Disease) cases/1,000/yr. Although about half of the cases had smoked less than 20 pack-years, smoking was the main risk factor for COPD, and it accounted for 29 to 39% of the new cases during the follow-up. Airway hyperresponsiveness was the second strongest risk factor (15-17% of new cases). Other determinants were respiratory infections in childhood and a family history of asthma, whereas the role of sex, age, and of being underweight largely depended on the definition of COPD used. COPD may start early in life. Smoking prevention should be given the highest priority to reduce COPD occurrence. Airway hyperresponsiveness, a family history of asthma, and respiratory infections in childhood are other important determinants of COPD. We suggest the need for a definition of COPD that is not exclusively based on spirometry.