Fahr's disease occurs in relation with many metabolic disorders especially with hypoparathyroidism. Imbalance of the coordination system and dysarthria were seen at the end of the treatment in a lung cancer patient treated with radiotherapy and chemotherapy. Fahr's disease was diagnosed by diffuse symmetric calcifications at white matter and basal ganglia of cerebrum and cerebellum in cranial computed tomography. Disease was thought to be caused by hypoparathyroidism with lower calcium and parathyroid hormone levels. Possible factor that caused hipoparathyroidism and also of Fahr's disease was radiotherapy performed to a wide area because of lung cancer. This case is the first Fahr's disease that was diagnosed concurrently with lung cancer.