An 80-year-old man who had undergone total gastrectomy and splenectomy for gastric cancer 13 years ago presented with headache, drowsiness, and high fever 1 month after a traffic accident. Brain CT scans revealed bilateral subdural fluid collections. Diffusion-weighted imaging (DWI) showed mixed high and low signal intensities in the left subdural fluid, and contrast-enhanced MR imaging revealed capsule enhancement of the left subdural fluid collection. The patient was diagnosed with left subdural empyema, and 2 burr-holes were drilled for drainage and irrigation. Operative findings revealed a neomembrane underneath the dura mater. Old hematoma and yellowish-white purulent fluid were present within the neomembrane. This confirmed the diagnosis of infected subdural hematoma (ISH). Abscess culture results were positive for Escherichia coli. The patient's symptoms resolved postoperatively with subsequent antibiotic therapy. However, 4 months after the operation, he suddenly died of severe sepsis and disseminated intravascular coagulation following cholecystitis, which was possibly associated with splenectomy. The clinical presentation, diagnosis, and treatment of an unusual case of ISH have been discussed. We emphasize that DWI and enhanced MR imaging may be useful for diagnosing ISH, and serial DWI evaluations may help in monitoring the therapeutic response in ISH.