IntroductionHistopathologically, herpes simplex virus type 1 causes hemorrhagic necrosis. Overt hemorrhage is infrequent in herpes simplex virus encephalitis but can lead to poor outcomes. This report describes a successfully treated case of herpes simplex virus encephalitis associated with subarachnoid bleeding in which real-time polymerase chain reaction was useful for diagnosis.Case presentationA 30-year-old previously healthy Japanese woman who had fever and headache for five days presented with disorganised speech, unusual behavior and delusional thinking. Real-time polymerase chain reaction amplification of herpes simplex virus type 1 in cerebrospinal fluid was positive (38,000 copies/mL) and antivirus treatment was started. During the course of her illness, the level of her consciousness decreased in association with desaturation and tachycardia. Thrombosis of the right pulmonary artery trunk with pulmonary embolism was evident on enhanced chest computed tomography. In addition, cranial computed tomography revealed subarachnoid and intraventricular bleeding. Intravenous heparin (12,000 U/day) was started and the dose was adjusted according to the activated partial thromboplastin time for about a month (maximum dose of heparin, 20,400 U/day). After the treatments, her Glasgow coma score increased and the thrombosis of the pulmonary artery trunk had disappeared.ConclusionsThe present case raises the question of whether anticoagulant treatment is safe in patients with herpes simplex virus encephalitis complicated by subarachnoid bleeding.