Object. For several decades, clinicians have predicted intraparenchymal brain pressure or brain tissue compression indirectly based on the degree of distortion of the midline structures (midline shift) and ventricle wall (ventriculomegaly) observed on conventional MRI. However, this method has several limitations. Diffusion tensor imaging (DTI) is a novel MRI technique that can provide information about the microstructural properties of compressed tissue. In this study, the authors evaluated whether DTI can precisely define the degree of tissue compression in patients with chronic subdural hematoma (CSDH). Methods. The study sample consisted of 18 patients (mean age 71 years, 10 men and 8 women) with unilateral CSDH and 12 age-matched volunteers. Diffusion tensor imaging results were acquired before and after the surgical irrigation in the CSDH group. Subdural pressure during the operation was also measured. Fractional anisotropy (FA) values were evaluated at several locations, including the gray matter. Results. The FA values of the gray matter, especially in the caudate nucleus and putamen, were increased in the patients with CSDH compared with the control group. The change in FA data before and after surgery (DFA) correlated with the degree of tissue compression evaluated by measurement of the subdural pressure. Furthermore, the increased FA values in patients with CSDH decreased after surgery. Conclusions. These findings indicate that FA values of the gray matter, especially in the caudate nucleus and putamen, may be important markers of tissue compression. The assessment of FA values of the gray matter will result in a new, less-invasive diagnostic technique to evaluate the degree of brain compression.