When you first heard of the name of "moyamoya disease" You may be upset when you first heard of the name of "moyamoya disease", which is unfamiliar even to your doctor. Moyamoya disease has another name: spontaneous occlusion of the circle of Willis. Thomas Willis (1621-1675) is the name of the British old famous physician and anatomist, who had studied the brain arteries. Blood supply to the brain is through the four large arteries, such as two carotid arteries and two vertebral arteries. These four arteries form the arterial circle at the base of the brain. This circle serves as an important collateral when one or more of them may occlude. In moyamoya disease (in the spontaneous occlusion of the circle of Willis), two carotid arteries occlude spontaneously at their terminal portions at the base of the brain. What happens next is the development of the collateral pathways to overcome a compromise of the blood flow. Normally, there are fine small arteries to bring arterial blood to the deep portion of the brain, which are called "perforating arteries". When these small perforating arteries have to carry more blood, they have to become enlarged. Thus, these arteries become prominent on angiography, which resemble the appearance of smoke of the cigarette in the air. In Japanese language, this appearance is expressed as "moyamoya", which means hazy, vague, and "not clear". This is a story of the name of the disease. The etiology of this disease is still unknown. In this sense, we think its etiology is also still in "moyamoya" state. On December 24, 2003: A Merry Christmas!