Thymoma masquerading as congenital partial absence of the left pericardium.

Interpretation of irregularities of the left heart border on routine posterior-anterior and lateral roentgenograms may be quite challenging. Partial absence of the left pericardium provides a characteristic abnormality of the left heart border. Described is a case of a thymoma which mimicked partial absence of the left pericardium on routine roentgenography. The importance of including both partial absence of the left pericardium and thymoma in the differential diagnosis of irregularities of the left heart border is discussed.