Rectus sheath hematoma should be included in the differential diagnosis of patients with acute abdominal pain. We describe three patients with rectus sheath hematoma, each of which produced a different image and was effectively diagnosed with ultrasonography. Case 1. Status asthmaticus in a woman aged 61 years who was admitted to this institution because of abrupt onset of dull abdominal pain in the lower left abdominal quadrant in a fit of continuous coughing. The pain increased in intensity during defecation several hours later. Abdominal examination detected a palpable mass accompanied by tenderness on the left side. The next day, the ultrasonogram showed a 74×60×24 mm hypoechoic mass under the left rectus abdominis muscle. Case 2, pain in the left lower quadrant in a man aged 75 years who was brought to the emergency room of this institution. The pain had begun during an attack of asthma and continuous coughing. Abdominal examination detected a palpable mass in the lower-left quadrant and was accompanied by tenderness, but the patient was discharged without being correctly diagnosed. The pain continued, however, and the ultrasonogram obtained 11 days later showed a 39×17 mm hypoechoic mass in the left rectus abdominis muscle. The ultrasonogram showed disappearance of the hematoma two months later. Case 3. A man aged 71 years abruptly complained of lower abdominal pain while hospitalized for a brain hemorrhage. His abdominal examination was significant for tenderness in the lower abdomen, but no palpable mass was detected in this area. The ultrasonogram obtained on the day of admission showed a 35×15 mm hypoechoic lesion in the right rectus abdominis muscle. We administered conservative treatment to all three patients. Ultrasonography proved useful for diagnosing and following up these cases of rectus sheath hematoma with ordinary medical history and physical examination.