The diffusion-weighted magnetic resonance (MR) imaging characteristics of chronic subdural hematoma and the correlation between hematoma liquidity and apparent diffusion coefficient (ADC) were investigated in 26 consecutive patients, 16 males and 10 females aged 42 to 92 years (mean +/- SD 73.3 +/- 13.1 years), with 31 chronic subdural hematomas. The chronic subdural hematomas were divided into homogeneous, separate, and trabecular types based on diffusion-weighted MR imaging findings. Almost all hematomas were low intensity on diffusion-weighted imaging, and the mean ADC value was 1.81 +/- 0.79 x 10(-3) mm2/sec. The high intensity areas in the subdural hematomas consisted of several types: high intensity line along the dura mater (subdural hyperintense band), high intensity along the intrahematoma septum, and laminar shape along the inner membrane. The subdural hyperintense bands accounted for almost all high intensity areas in the subdural hematomas. The mean ADC value of the high intensity areas was 0.76 +/- 0.24 x 10(-3) mm2/sec, close to that of the normal brain. The subdural hyperintense bands were considered to be intracellular and/or extracellular methemoglobin based on the T1- and T2-weighted imaging and intraoperative findings. The subdural hyperintense band is an important finding indicating relatively fresh bleeding from the outer membrane. Diffusion-weighted imaging shows liquid subdural hematoma as low intensity, and measurement of the ADC values can differentiate between liquid and solid components of the chronic subdural hematoma.