Perfusion and ventilation lung scintigraphy are indicated: (1) for evaluating regional perfusion and ventilation, (2) for establishing a ratio between ventilation and perfusion in different areas of the lung, (3) when chest surgery is planned (especially for mapping out the extent of resection), (4) when surgery of the upper abdomen is to be performed, and (5) when global function studies are compromised. Regional ventilation and perfusion studies are invaluable in the diagnosis of pulmcnary thromboembolism in which a perfusion defect is accompanied by well-preserved ventilation in the underperfused area. Regional function tests are indicated for evaluating the distribution of ventilation and pulmonary circulation in practically any chronic pulmonary disease, especially when global function studies, such as spirometry, show significant decreases.