Background: Rectus sheath hematoma (RSH) is a rare entity that can mimic an acute abdomen. Therefore, we designed a study to analyze the etiology, frequency, diagnosis using ultrasound, and treatment of RSH. Methods: A total of 1,257 patients admitted for abdominal ultrasound for acute abdominal pain or unclear acute abdominal disorders were evaluated. Results: In 23 (1.8%) patients, an RSH was diagnosed; three of them were not diagnosed preoperatively by ultrasound. Of 13 men and 10 women (mean age, 57 +/- 23 years), 13 developed RSH after local trauma, three after severe coughing, two after defecation, and five spontaneously. Fifteen had nonsurgical therapy, and eight underwent surgery. The use of anticoagulants was accompanied by a larger diameter of the RSH (p <.012), and surgical therapy was more frequently required in these patients. In the surgically treated group, more intraabdominal free fluid could be detected by ultrasound (p <.0005), patients required less analgesics (p <.001), and the mean hospital stay was shorter (p<.001). Conclusions: RSH is a rare condition that is usually associated with abdominal trauma and/or anticoagulation therapy. Ultrasound is a good screening technique. Nonsurgical therapy is appropriate but leads to a greater need for analgesics. Surgery should be restricted to cases with a large hematoma or free intraabdominal rupture.