Epidemiological studies over the last thirty years have suggested that lower respiratory tract infection (LRTI) in the first few years of life may be involved in the aetiology of chronic airflow obstruction (CAO). However, evidence for a causal relationship has been limited. Discovery of child health records for men and women born in Hertfordshire in 1911-1930 and in Derbyshire in 1917-1922 provided the opportunity to examine this relationship in elderly individuals. A follow-up study of 845 men who were born and still living in East Hertfordshire showed that men who had had bronchitis or pneumonia in infancy had significantly lower lung function at 64 years of age than other men, after allowing for age, height and smoking history. A follow-up study was conducted to assess the lung function of men and women aged 67 to 74 years who were born and still living in South Derbyshire. Subjects were recruited through nine General Practices and 330 men and 288 women with complete health visitor records were interviewed and examined. Men who had had pneumonia in the first two years of life had a reduction in mean Forced Expiratory Volume in one second (FEV₁) of 0.76 litres (95% CI:0.41 to 1.11 litres; p < 0.0001), after allowing for age, height and smoking. They had a higher prevalence of CAO, defined as a ratio of FEV₁/Forced Vital Capacity ≤ 60%, than other men (odds ratio adjusted for smoking: 7.3; 95% CI: 1.9 to 28.4; p=0.004) and they had a higher mean serum level of elastin-derived peptides (p=0.001). In women, pneumonia in the first years of life was associated with a smaller and non-significant reduction in lung function. The findings in men suggest that pneumonia in the first two years of life is causally related to CAO in late adult life and that it may lead to destructive emphysema.