Seventy-five patients with moyamoya disease were admitted at Kyung Hee University Hospital from 1978 to 1993. Of these, we followed up fourty-five cases who had been treated conservatively for the study of the natural history of moyamoya disease. Seven operated cases and six dead patients during admission were excluded in this study and sixteen cases were lost to follow-up. There were 16 males and 29 females. The hemorrhagic group was twenty-three cases and the non-hemorrhagic group was twenty-two cases. On angiographic findings, the typical type was twenty-eight cases and the atypical type was seventeen cases. Twenty-one cases(91%) of the hemorrhagic group were over twenty years old and sixteen cases(73%) of the non-hemorrhagic group were under twenty years old. The mean follow-up period was 66 months(range 6 months to 15 year 4 months). Nineteen cases had reattack, but most of them had good result except only two cases who died due to huge intracerebral hematoma. Of the sixteen cases, who were under twenty years old and manifested as transient ischemic attack, fifteen cases were improved only with conservative management. The good result of fourty-one cases out of fourty-five with conservative management. suggests that spontaneous collateral circulation had developed well to restore the function of ischemic brain in the natural course of moyamoya disease. Moyamoya disease seems to be not so worse in natural course than expected and further study about natural history will be necessary.