In this study, measurement of Epstein-Barr virus (EBV) and cytomegalovirus (CMV) viral antibody titers and analysis of both viral genomic sequences using polymerase chain reaction (PCR) were performed to clarify the correlation of viral infection and Moyamoya disease. Serum samples were obtained from 64 patients with Moyamoya disease. The ages ranged from 5 to 66 years, with a mean age of 35.1 years. There were 23 males and 41 females. The serum antibody titers to CMV and EBV were measured by means of compliment fixation test and fluorescent antibody method respectively. Those titers of the patients were compared with those of 13 patients of atherosclerotic internal carotid occlusion and 34 normal volunteers. On the other hand, CMV and EBV genomic sequence using PCR, which was utilized with specific primer pairs, were performed in 22 patients of Moyamoya disease and ten normal volunteers. The following results were obtained; The antibody titer of EBV in Moyamoya disease was significantly higher than that in controls. However, no significant difference of antibody titer against CMV was detected. In EBV DNA analysis by use of PCR, EBV DNA was proved in 15 out of 20 patients with Moyamoya disease and four out of nine normal controls. Namely, EBV DNA was seen more frequently in patients with Moyamoya disease, compared with normal controls. In inverse, CMV DNA was not seen in patients with Moyamoya disease nor normal controls. In conclusion, the antibody titer of EBV revealed high levels in Moyamoya disease and EBV DNA was also detected more frequently in patients with Moyamoya disease. These results suggested a possibility that EBV infection might be involved in the pathogenesis of Moyamoya disease.