Patient: Female, 74 Final Diagnosis: Iliopsoas abscess Symptoms: Fever Medication: - Clinical Procedure: - Specialty: Infectious Diseases Objective: Rare disease Background: While uncommon, iliopsoas abscesses can become the underlying cause of a fever of unknown origin. Even in such cases, it is considered rare for an iliopsoas abscess to extend into the subcutaneous space. Case Report: A 74-year-old woman with a history of schizophrenia was referred to our hospital with a high-grade fever. The patient was unaware of her febrile status prior to admission. There was no previous hospital admission. Examination revealed a non-tender mass in the lower right back that the patient had been aware of for approximately 1 month. Initially, we considered a subcutaneous abscess; however, computed tomography (CT) detected a large mass in the right retroperitoneum, which extended into the adjacent subcutaneous space. Surgical drainage was performed. M. morganii was detected in fluid evacuated from the abscess and in a urine culture. Blood cultures were negative. A repeat enhanced CT revealed a right renal abscess with staghorn calculus. This iliopsoas abscess was considered to be due to a renal abscess. The combination of a minimally aggressive bacterial species and the absence of disease awareness resulted in uncontrolled abscess growth in this case. Surgical drainage and salvage nephrectomy was subsequently performed, and she was discharged to a nursing home. Conclusions: M. morganii can lead to massive abscess formation without an underlying immunocompromised status. Iliopsoas abscesses can surreptitiously extend into the subcutaneous space; therefore, not all abscesses observable from the surface are necessarily subcutaneous in origin.