Pseudothrombocytopenia caused by platelet clumping is an in vitro phenomenon that occurs in ethylene-diamine-tetra-acetic acid(EDTA) anticoagulated blood at room temperature. Pseudothrombocytopenia may lead to erroneous diagnosis, unnecessary and costly additional laboratory examinations, and inappropriate medical or surgical therapy. We experienced 75-year old female pseudothrombocytopenia patient scheduled for orthopedic surgery, who showed abnormal thrombocytopenia (35,000/mm3) in preoperative routine platelet count using EDTA anticoagulant, but showed normal platelet count in sodium citrate anticoagulant. (Korean J Anesthesiol 1998; 34: 213∼216)