The patient was a 21-year-old woman who, at 16 years old, had had no subjective symptoms. She was referred to our hospital because of abnormal shadows in the middle of the left lung that were seen on a chest radiograph during a school medical screening. In her first examination at our hospital, CT revealed tumor-like shadows with diameters of 2.0 cm in Lt-S6 and 0.5 cm in Rt-S5, together with arteries and veins. Lung ventilation and blood flow scintigraphy was used to obtain images of both kidneys and the head, and as a result, a pulmonary arteriovenous malformation (PAVM) was diagnosed. Because of a low shunt fraction of 10.8%, the patient was only kept under observation. However, at 21 years of age, she complained of dyspnea with exercise and was admitted to the hospital for further examination. Chest CT showed an increase in the diameter of the arteriovenous malformation (AVM) to 2.7 cm, and 3-D image reconstruction clearly revealed the position of the tumor in relation to the pulmonary artery and vein. The shunt fraction had increased to 18.3%. Coil embolization was therefore performed to A6b, the artery feeding the AVM (using interlocking detachable coils of 6 mm-10 cm and 8 mm-20 cm, and Trufill coils of two 7 mm-6 cm and 5 mm-4 cm). The patient had no serious postoperative complications. A decreased AVM diameter of 1.7 cm and an improved shunt fraction of 11% were observed, and she was discharged from the hospital. Since then, she has been well, without dyspnea, and her chest films show only coils and no aggravation of other AVMs. AVMs generally have no symptoms, but they can cause severe complications such as hypoxia from a right-to-left shunt, hemoptysis, hemothorax, and brain abscesses. Coil embolization should be considered more often for the treatment of AVM because it is safer, more effective and less invasive than traditional operations.