A 67-year-old woman was on LMWH (enoxaparin) therapy because of atrial fibrillation. At the third day of treatment, an abdominal pain, especially on the left side, was reported by the patient. On physical examination, a large solid mass with a size of 10 x 15 cm was palpated in the upper and lower left quadrants of the abdomen. Laboratory tests revealed a decrease in hemoglobin level by 5.8 gr/dL. Abdominal ultrasonography showed a large superficial cyst-like mass on the abdominal wall. On CT scans, a large iso- to hyperdense mass (7x16x14 cm) with hypodense areas in it posterior to the rectus abdominis muscle was demonstrated. LMWH treatment was discontinued, and 2 sets of red blood cell replacement were done. At next day, volume of the mass decreased and abdominal ultrasonography showed a smaller hematoma. Abdominal pain was resolved at the sixth day. She was discharged at the eighth day.