To clarify the current clinical features of probable Moyamoya disease in Japan, 180 cases were analyzed based on the cases collected by the Research Committee on Spontaneous Occlusion of the Circle of Willis of the Ministry of Health and Welfare, Japan. Although the age distribution patterns of probable Moyamoya disease showed two peaks similar to that of definite cases, the pattern shifted to the right thus indicating a reversed children versus adult ratio. The female/male ratio was 1.65, which was not significantly different from that observed in definite cases. The family occurrence rate was 6.7%. The incidence of cerebral infarction and bleeding in probable cases was higher than that in definite cases. The incidence of hemorrhage was higher in females than in males. Angiographically probable cases were at earlier stages in comparison to definite cases. The rebuild-up phenomenon in electroencephalograms was less detectable than in definite cases. Surgical procedures were performed in 63.3% of all cases and approximately 81% of them underwent bypass surgery. Single encephalo-duro-arterio-synangiosis, direct bypass and combined bypass procedures were performed in 35, 40 and 25% of all surgical cases, respectively. Although the incidence of bleeding was higher in the probable cases, the outcomes of the patients were similar to that of the definite cases except for mortality. Approximately 7% of the probable cases developed into definite type within an average 6.6 year follow-up period. The majority of probable Moyamoya disease cases thus seems to have somewhat different clinical characteristics from the definite disease cases.