In this paper, abdominal ultrasound examination as a point-of-care examination (POCUS) is discussed. POCUS is very useful in various clinical situations, especially for the diagnosis of critically ill patients with non-specific symptoms. In patients with an unknown fever origin, POCUS can detect unexpected infection foci such as liver abscesses. Pseudomembranous colitis, which is one of the important causes of fever during/after the use of broad-spectrum antibiotics and sometimes difficult to diagnose or even suspect as the cause of fever, can also be diagnosed with POCUS. Malignancies such as malignant lymphoma are also frequent causes of fever of unknown origin and, with POCUS, we have diagnosed many cases with malignant tumors presenting only with low-grade fever. Abdominal fullness is another common symptom of critically ill patients. POCUS is very useful for differentiation among several diseases causing abdominal fullness. Ascites is expressed as anechoic fluid in the abdominal cavity, while bowel obstruction as bowel distention presenting the so-called keyboard sign. Urinary retention, often misdiagnosed as bowel obstruction or an abdominal tumor, is also easily diagnosed by POCUS. It is very difficult to detect the occult causes of exacerbation of the general condition and/or markedly abnormal laboratory data in critically ill patients under endotracheal intubation or the influence of sedative agents, which make it difficult for the patients to complain of their symptoms. Attending physicians should make the best of POCUS as a useful diagnostic modality for these patients.