Fifteen years of experience with adult moyamoya disease (MMD) in a single department were analyzed to describe the clinical features and to clarify the existence of the disease entity of adult MMD with special reference to its diagnosis. This study included 348 patients treated between 1996 and 2010. Male to female ratio was 128 to 220 with mean age of 39 years. The presence of neurological symptoms and signs during childhood and mean Suzuki stage according to the age were evaluated. Adult MMD was differentiated from other diseases, such as vasculitis, by comparing the laboratory data including common markers and specific markers for such diseases between 22 adult MMD and 21 atherosclerotic disease (ASD) patients. Conventional angiographic data were analyzed to detect progression in 52 unilateral adult MMD patients over a mean follow-up duration of 4.8 years. Only one of 128 patients (0.8%) aged over 40 years showed childhood symptoms and signs. The distribution of the mean Suzuki stage between patients aged 40 years and under and over 40 years was similar. Laboratory tests differentiating the other diseases with the same phenotype with MMD revealed no difference between adult MMD and ASD. The progression of unilateral MMD to bilateral MMD occurred in 6 of 52 patients (11.5%). Adult MMD is not a syndrome, but a distinct disease entity with significant progression of unilateral MMD to bilateral MMD. Diagnostic and therapeutic strategies should be different from pediatric cases when considering adult MMD.