The retrofascial space is a concealed one located behind the transversalis fascia in the posterior peritoneal cavity. It contains mainly muscles and fasiae and extends from the mediastinum to the pelvis. Clinical manifestations of an abscess are usually obscure except for hip joint problems when the abscess developed. Computed tomography is done immediately if an abscess is suspected, and there have been good results under incision with drainage and antibiotics. It is different from retroperitoneal abscess anatomically, pathologically, bacteriologically and clinically, so we report a case for reference. The patient, a 43-year-old female worker, had suffered from disability of her left lower limb for more than 5 months before admission. She even visited her local medical doctor, but in vain. In the week prior to admission, left hip swelling with awkwardness in walking was felt, and she called on an orthopedist. After brief examination, impaired hepatic and renal function was noted, so she was referred to our hospital. The chief complaint on admission was left hip swelling and pain with radiation to the inguinal and knee area for one week. The past history was insignificant except for a cesarean section and an appendectomy done more than 10 years before. Upon physical examination, the patient was found to be pale, emaciated, mildly icteric and had a flexion contracture of the left thigh. Laboratory data revealed anemia, leukocytosis, impaired hepatic and renal function. The chest X-ray was normal, and KUB demonstrated left hip joint flexion and external rotation. A sonogram showed a localized hypoechoic lesion in the left posterior peritoneal cavity 2 days later.(ABSTRACT TRUNCATED AT 250 WORDS)