RATIONALE: Few studies have investigated the factors associated with the early inception of chronic obstructive pulmonary disease (COPD). OBJECTIVE: We investigated COPD risk factors in an international cohort of young adults using different spirometric definitions of the disease. METHODS AND MEASUREMENTS: We studied 4,636 non-asthmatic subjects who had pre-bronchodilator FEV1/FVC measured in the European Community Respiratory Health Survey both in 1991-1993 (when they were 20-44 years old) and in 1999-2002. COPD was defined according to the GOLD fixed cut-off criterion (FEV1/FVC<0.70), and two criteria based on the Quanjer and LuftiBus reference equations (FEV1/FVC