Moyamoya disease (MMD) is a spontaneous occlusive disease of the Willis circle. This study evaluated the presence of the radiological hallmarks of adult MMD on a "brain check-up" (BC) using MRI and MR angiography, in Japan. BC was offered to 11,402 healthy subjects (7570 men and 3832 women) between January 1997 and November 2003. The mean age of all subjects was 53.2 years (SD 11.1); the mean age of men was 53.2 years (SD 10.9) and women 53.2 years (SD 11.5). MMD was diagnosed according to the criteria of the Research Committee on Spontaneous Occlusion of the Circle of Willis in Japan. Eight subjects (4 men and 4 women) were diagnosed as probable MMD without neurological deficits. Thus, the percentage of subjects with asymptomatic MMD on BC was 0.07% for the total group, 0.05% in men and 0.10% in women. The female to male ratio was 3.3:1. The mean age of the MMD patients was 54.0 years (SD 12.0); men 54.8 years (SD 12.8) and women 53.3 years (SD 13.9). Estimates of the prevalence of MMD in the Japanese population were calculated as 50.7 per 100,000 people; 28.9 in men and 94.3 in women. Five patients had a family history of definite MMD and seven had a family history of subarachnoid or intracerebral hemorrhage. MRI and angiography showed stenosis of the bilateral internal carotid arteries (ICA) in five patients and a unilateral ICA in three. Moyamoya vessels were confirmed on cerebral angiography. Our standard BC protocol is limited for the detection of moyamoya vessels in the initial stages of MMD. The present study suggests that the radiological features of MMD are asymptomatic in the early stages of the disease.