We present the case of a 64-year-old female treated surgically for ligamentum flavum hematoma that caused progressive radiculopathy. Initially, she suffered from an acute onset of lower back pain. Only a history of minor back injury was discovered. She rapidly became unable to walk. Magnetic resonance imaging demonstrated an epidural mass lesion at L3 to L4 that was continuous with the left ligamentum flavum. The mass was hypointense on T1-weighted images and centrally hyperintense and marginally hypointense on T2-weighted images. The margin was well enhanced by gadopentetate dimeglumine administration. After removal of the mass, the patient's symptoms completely resolved. Before surgery, we believe accurate diagnosis of ligamentum flavum hematoma can be based on magnetic resonance imaging.