Objective To study the changes in pulmonary diffusing capacity (DL), alveolar capillary membrane diffusing capacity (Dm) and pulmonary capillary blood volume (Vc) in patients with severe acute respiratory syndrome (SARS), and to elucidate the underlying pathophysiogical mechanism of reduction of pulmonary diffusing capacity. Method Spirometry was performed in 26 SARS patients and 12 healthy individuals in resting condition. DLco were measured by single breath method, estimations of Dm and Vc were done by the method of Roughton and Forster. Results DLco in SARS patients was significantly lower than that in normal control, the same was true for Dm and Vc. The severer the pulmonary lesion, the heavier the damage to the pulmonary diffusing funetion. Conclusion The changes in Dm and Vc were both found in patients with SARS. Their measurements were helpful for detecting pulmonary involvement in SARS and defining the reason of DL abnormality in SARS. Dm and Vc were important and sensitive for monitoring pulmonary diffusing function in SARS patients