Basal moyamoya vessels are a potential source of hemorrhage in patients with moyamoya disease, but the etiology remains unclear. Symptomatic hemorrhage resulting from long-standing hemodynamic effects on pathologically dilated, fragile moyamoya vessels may be preceded by asymptomatic microbleeding in adult moyamoya disease patients, regardless of hemorrhagic or ischemic onset. T2*-weighted magnetic resonance (MR) imaging was used to investigate the presence of microbleeds in 27 adult patients with angiographically confirmed moyamoya disease, 21 females and six males aged 18-70 years (mean 40.8 +/- 15.7 years). Clinical diagnosis was intracranial bleeding in six patients, transient ischemic attack or cerebral infarction in 18, and asymptomatic in three. Asymptomatic microbleeds were detected in four of the 27 patients, two of six who initially presented with hemorrhagic events and two of 18 with ischemic onset. These microbleeds were located in the paraventricular white matter, temporal subcortex, and basal ganglia. The presence of microbleeds had no correlation with either patient age or duration from disease onset or diagnosis of disease. A large cohort study is needed to explore the significance of asymptomatic microbleeds in moyamoya disease.