Rationale Following preterm birth, limited data exist in adulthood regarding alveolar-capillary growth and exercise capacity. Gas transfer at rest through exercise is a non-invasive measure of alveolar-capillary development. Objectives To determine exercise capacity and gas transfer at rest and during exercise in ex-preterm adults. Methods Ex-preterm subjects (n=60, median gestation 31.5 weeks) recruited at birth underwent exercise testing at 21 years of age, and were contemporaneously compared with 50 healthy controls. Measurements Subjects exercised on a cycle ergometer and measurements of heart rate (HR), functional residual capacity (FRC), effective pulmonary blood flow (Qpeff), stroke volume (SV), carbon monoxide transfer (DL_CO), oxygen consumption, (VO₂), arteriovenous oxygen difference (AVO), transit time (TT), respiratory rate (Rf), carbon dioxide excretion (VCO₂), minute ventilation (VE), tidal volume (V_T), and respiratory quotient (RQ) were made using a respiratory mass spectrometer. Main Results In the index study group and controls, the median (95% confidence interval [CI]) DL_CO (mmol/min/KPA/m²) at rest was 4.33 (4.18 to 4.62) and 4.75 (4.50 to 5.10) respectively (p=0.01) and the median (95% CI) Qpeff (litres/min/m²) at rest was 3.26 (3.16 to 3.49) and 3.59 (3.43 to 3.81) respectively (p=0.04). Both Qpeff and DL_CO values normalized during exercise but were again reduced and lower than controls following a recovery period. No significant differences were found in exercise capacity between the groups. Conclusions The data suggest a long term effect of premature delivery on resting cardiac output and gas transfer, not due to abnormal cardiac or pulmonary function and with no evidence of exercise limitation.