A 78-year-old man who had a history of myocardial and cerebral infarction and who was treated with aspirin and warfarin, presented with left chronic subdural hematoma. Cerebral computed tomography showed severe brain compression of hematoma with midline shift, indicating the need for emergent surgery. The hematology and clotting tests upon admission revealed severe thrombocytopenia (platelet count, 1.3 × 10(4)/μL) with normal clotting activity. Because platelet aggregation was evident in the smear, we re-examined the patient for hematology using tubes that contained heparin, showing also low platelet count (2.3 × 10(4)/μL). The day on admission, we performed irrigation and drainage of the chronic subdural hematoma through single burr-hole craniostomy. During surgery, we used 10 units of platelet concentrates (PCs) for the reason that the patient was taking aspirin and coagulopathy derived from low platelet count could not be excluded. After surgery, we re-evaluated the hematology of the blood stored in tubes that contained ethylenediaminetetraacetic acid (EDTA) with or without kanamycin (KM). Treatment with KM dissociated EDTA-induced platelet aggregation and revealed platelet counts with highest accuracy (no KM treatment, 1.3 × 10(4)/μL; KM treatment, 15.2 × 10(4)/μL). This phenomenon is called EDTA-Dependent Pseudothrombocytopenia (PTCP) defined as falsely low platelet counts reported by automated hematology analyzers due to platelet aggretgation. Awareness of the phenomenon will enable neurosurgeons to manage patients with PTCP appropriately and clinical laboratory especially in emergency hospital is recommended to prepare for the hematological tubes being added KM in routine analysis, resulting in preventing mistaken diagnosis.