An unusual isodense acute subdural hematoma is reported. Its causes may have included dilution with cerebrospinal fluid and a local or disseminated coagulopathy. Detection of such lesions requires a high incidence of suspicion based on subtle abnormalities found on computed tomography and the use of enhancement, particularly with the aid of advanced scanners. When a significant shift of the midline is seen, other alternatives would be angiography or placement of an exploratory burr hole on the "swollen" side.