A 55-year-old man was transported to our hospital after a sudden onset of left lower abdominal pain while driving. Computed tomography (CT) of the abdominal region revealed an extensive iso-intense signal region that had a maximum area of 14×15 cm, which we treated conservatively. A series of follow-up CT images showed the gradual decrease of the left peritoneal mass, while continuity with the left adrenal gland became apparent. He was diagnosed with idiopathic adrenal hemorrhage. Adrenal hemorrhage presenting with huge retroperitoneal tumors is rare, and most cases are treated surgically. Therefore, CT images with conservative treatment are rare, holding both clinical interest and significance.