Venous thrombus was recognized in the upper extremity of a 53-year-old man after blood donation. The patient presented with a 15-day history of swelling in the left upper-extremity that started 6 hours after apheresis. Contrast-enhanced computed tomography revealed clots in the deep veins of the left arm and the peripheral pulmonary artery. Blood donation had proceeded smoothly, and the patient had no thrombotic predisposition, except for a smoking habit. The thrombus resolved following anticoagulant therapy, and the patient' s clinical course was uncomplicated. Despite a thorough investigation, the cause of this thrombus remains unknown.