OBJECTIVE To determine the cut-off value for distinguishing a normal versus an abnormal right-to-left shunt percentage on lung perfusion scintigraphy using (99m)Tc-macroaggregated albumin (MAA). METHODS Fifty-three patients (eight patients with a right-to-left shunt and 45 without a right-to-left shunt) who underwent MAA whole-body imaging for the evaluation of right-to-left shunts were divided into group 1 (eight patients with brain MAA uptake) and group 2 (45 patients without brain MAA uptake). Moreover, group 2 was subdivided into two categories (groups 2a and 2b) based on the results of lung computed tomography, electrocardiography examinations, and pulmonary function tests. The average and standard deviation (SD) of each group were compared. In addition, we estimated the cut-off value for a normal right-to-left shunt percentage using whole-body imaging. RESULTS The average right-to-left shunt percentage values and SD were 23.67±12.17% in group 1, 6.68±1.04% in group 2a, and 6.60±0.84% in group 2b. The shunt percentages of groups 2a and 2b were not significantly different (P=0.77). The estimated normal value (mean±2 SD) of group 2 was 6.64±0.94%. Meanwhile, the cut-off value was estimated as 10% based on the distributions of MAA shunt percentages for groups 1 and 2. CONCLUSIONS The normal range (mean±2 SD) was 6.64±1.88%. The cut-off value for the normal right-to-left shunt percentage in MAA scintigraphy was 10%.