The radioisotop study using 99mTc-macroaggregated Albumin is a simple, non-invasive test for the diagnosis of pulmonary arteriovenous fistula(PAVF). It can show the presence . of the right-to-left shant at the lung level using the dynamic study, and also the shunt fraction can be estimated. Here we presented the results of the radioisotope study on two patients with PAVF. In one patient, the cardiac catheterization was done and calculated shunt fractions by both method were well matched.