Pulmonary arteriovenous malformation (PAVM) is an uncommon abnormality, which has variable manifestations relating to the degree of shunting and is usually diagnosed by chest radiography. However, some diffuse type PAVMs can only be seen at the microscopic level, neither by a chest plain film nor a computed tomography. A 12-year-old girl had symptoms of cyanotic lips and exercise intolerance for seven years without a definite diagnosis despite several examinations. Another 7-year-old girl was incidentally noted to have lip cyanosis when crying during her hospitalization for a crushed finger. These two patients were suspected to have diffuse type PAVMs and lung perfusion scans confirmed the diagnosis.