Hematomas caused by surgery or trauma that persist and expand slowly for more than a month are defined as chronic expanding hematomas (CEH). Magnetic resonance imaging (MRI) is useful for the diagnosis. Total excision with the pseudocapsule is the treatment method. Pseudoaneurysms result from arterial wall disruptions and can be mistaken for CEH. We present a rare case report of a 45-year-old man with a large, painful swelling in his left popliteal fossa. He had a puncture wound by a nail 11 years ago and a gradually expanding mass occurred in his popliteal fossa. A pseudoaneurysm was detected and operated a year later. After surgery, a gradually expanding mass recurred in his popliteal fossa. On the arteriography, the popliteal artery was occluded and the blood flow was maintained with collateral vessels. On MRI, an enormous swelling of 115 × 107 × 196 cm in diameter was seen. It was diagnosed as CEH and was excised completely protecting the collateral vessels and there was no recurrence after a year from the surgery.