Lower respiratory infections in early childhood, asthma and wheezy syndromes not diagnosed asthma, allergy, airway hyperreactivity, and passive and active tobacco smoke exposure are all related to the level of small airway function measured by spirometry (average expiratory flow rate during the middle half of the exhaled force vital capacity [FEF 25-75] and expiratory flow rate at 50% of the exhaled forced vital capacity [V_max-50] analyzed most often) in middle childhood. There is a strong tendency for children to maintain their relative levels of spirometric performance through childhood into adolescence and young adulthood (tracking). Therefore, factors that are related to the level of lung function in early to middle childhood are also associated with functional status in late childhood, adolescence, and young adulthood. Since starting level of lung function in early to middle adulthood is related to rate of loss of lung function in later adulthood, childhood factors have the potential for influencing patterns of decline in lung function in adulthood and the risk of developing symptomatic obstructive airway disease in later life.