The variable and nonspecific presentations of psoas abscess, as well as its infrequent incidence in the emergency department (ED), can result in delayed diagnosis or misdiagnosis. Previous reports have not discussed the diagnostic difficulties of psoas abscess from the viewpoint of emergency physicians (EPs), especially in light of the widespread use of ED ultrasonography. This report describes a 1-year experience between November 1993 and October 1994, during which 10 ED patients were diagnosed to have psoas abscess; in 7 cases, diagnoses were established in the ED. Patients' mean age was 64.6 years (range, 46 to 76). Pain was the most frequently encountered symptom (80%), with 5 patients (50%) complaining of flank pain. The triad of fever, flank pain, and limitation of hip movement, which is specific for psoas abscess, was present only in 3 patients (30%). The mean duration of symptoms was 10.6 days (range, 1 to 30 days). The mean time spent to establish the diagnosis was 1.7 days (range, 0 to 7 days). The diagnosis of psoas abscess was established by ultrasound in 6 patients, by computed tomography (CT) in 3 patients, and by surgery in 1 patient. Four patients who presented with either sepsis and nonspecific abdominal/flank pain or sepsis and thigh swelling were diagnosed to have psoas abscess by ultrasound performed by EPs. Only 3 patients were admitted to the ED with an initial diagnosis of psoas abscess. The remaining 7 had the following initial ED diagnoses: 2, fever of unknown origin; 2, septic shock; 1, shock; 1, sepsis; and 1, peritonitis. All but one had manifestations of sepsis. Two patients died of septic shock; these two patients had failed to be drained well. This report also includes a discussion of the role of EPs and ultrasonography in the diagnosis of psoas abscess. With their alertness and their expertise in ultrasonographic techniques, EPs can make an immediate diagnosis and arrange an early drainage procedure. For patients with sepsis of unknown origin, prolonged fever of unknown origin, and some specific manifestations suggestive of psoas abscess, the screening ultrasound should scan not only abdominal solid organs but also peritoneal cavity and retroperitoneal space. In addition, a flow chart is presented for facilitating the diagnosis of psoas abscess in the ED.