This case report describes an 84-year-old female patient with a growing chronic subdural hematoma (CSDH) that showed an enhanced subdural hyperintense band on magnetic resonance diffusion-weighted imaging over time, indicating that it reflects fresh bleeding from the outer membrane covering the hematoma. The presence of subdural hyperintense band on magnetic resonance diffusion-weighted imaging of CSDH, possibly reflecting hemorrhagic diathesis of the outer membrane, might be a predictor of enlargement of CSDH.