Mild traumatic brain injury is common in elderly patients, many of whom are on anticoagulant. The common practice is to discharge these patients from the emergency room if the computed tomography (CT) of the brain is normal. However, a very small proportion of these patients may develop a life threatening intracranial haematoma in the following days. We present here a case of a 66-year-old male on anticoagulant therapy that developed a subdural haematoma 48 hours after a mild head injury, with a normal initial CT scan of the brain. The patient underwent a craniotomy with evacuation of a large subdural clot. Postoperatively he had progressively improved and six months later has a Glasgow Outcome Score of three. This case is characterized by the delayed onset of a subdural haematoma in a patient on anticoagulation and we discuss here the possible pathogenesis related to this phenomenon. We also briefly review the pertinent literature and the current guidelines for the management of this type of head injuries.