OBJECTIVE: To investigate the associations of birth weight, body mass index (BMI) during childhood and youth, and current BMI with adult lung function. DESIGN: Population-based longitudinal study of juvenile obese and non-obese men, who were identified at draft board examination (age range: 19-27 y) and who participated in a follow-up examination in 1981-1983 (age range: 25-48 y). Birth weight, childhood weight and height measurements from 7 to 13 y of age were obtained from school health records. Current BMI and lung function were assessed at follow-up. SETTING: Copenhagen and adjacent regions, Denmark. SUBJECTS: In total, 193 juvenile obese men at draft board examination and 205 randomly selected nonobese controls from the same population. MAIN OUTCOME MEASURES: Lung function measured by forced expiratory volume in 1 s (FEV(1)) and forced vital capacity (FVC), adjusted for age and height. RESULTS: After adjusting for current BMI, smoking and education, birth weight was positively related to FEV(1), although only with borderline statistical significance. BMI at age 7 y was positively associated with both FEV(1) and FVC, whereas BMI at later ages in childhood and in youth was not associated with these measures. There was a strong negative linear relation between current BMI and lung function among those currently overweight and obese (BMI 25 kg/m(2)), whereas no association was seen in the non-obese (BMI <25 kg/m(2)). CONCLUSION: Our findings confirm the detrimental effect of high current BMI on adult lung function, and further suggest that early childhood growth has a protective influence.