Rationale There is an association between childhood chest illness and impairment of adult ventilatory function. It has not yet been established whether respiratory illness in childhood predisposes to chronic obstructive pulmonary disease by accelerating the decline in adult lung function. Objectives To determine the effects of childhood chest illness and smoking on the rate of decline of adult ventilatory function from the age of 35 to 45 years in a large, nationally representative sample of British adults. Methods Spirometry measurements were compared at 35 and 45 years of age in 1158 adults participating in the British 1958 Birth Cohort. Multiple regression analysis was used to measure the association between childhood chest illness and within person change in spirometric volumes between age 35 and 45 years, adjusting for potential confounding factors. Measurements and Main Results The mean reduction in 1-second forced expiratory volume between ages 35 and 45 years was 35 ml/yr. Compared to subjects without the relevant respiratory history, the rate of decline was not significantly associated with pneumonia (mean difference -0.2, 95% confidence interval -6.1 to +5.8ml/year), whooping cough (0.7 (-5.1 to +6.5ml/year), wheeze by age 7 years (0.4 (-5 to +5.9ml/year)), or wheeze onset age 8 to 16 years (-3.4 (-10.5 to +3.6ml/year). A similar pattern emerged for forced vital capacity. Conclusions Childhood chest illness does not adversely affect the rate of decline of lung function in mid-adult life.