Chronic obstructive pulmonary disease is a major health burden. Evidence that childhood lower respiratory tract infection (LRTI) is associated with reduced adult lung function and thereby with chronic obstructive pulmonary disease comes from 3 sources. First, studies of children hospitalized with specific LRTIs, for example, as a result of respiratory syncytial virus, show reduced lung function 7 to 10 years later, but many have diagnostic and referral biases. Second, population studies show that adults reporting childhood LRTI have reduced lung function, but retrospective ascertainment of LRTI is unreliable. Finally, in the largest study of adults with independent ascertainment of childhood LRTI, those with pneumonia before age 7 years had a 6% to 7% lower unadjusted mean forced expiratory volume in 1 second and forced vital capacity. The deficits in adjusted lung function persisted after albuterol was administered and were neither due to wheezing illness nor diminished after results were controlled for confounders. Loss of lung function was no greater in those with pneumonia at age <2 years than in those with pneumonia at age 2 to 7 years. This and similar studies strongly support an association between childhood pneumonia and a reduction in adult lung volume, whereas follow-up studies of children with specific LRTIs show an obstructive defect. Ongoing studies that have ascertained premorbid lung function should help determine whether pneumonia causes this deficit or is commoner in those with poorer premorbid lung function.