Pyogenic sacroiliitis is a rare infection that may accompany psoas muscle abscess with vague clinical presentation, which present a diagnostic challenge requiring a high index of suspicion. We report a 48-year-old male, diagnosed pyogenic sacroiliitis with psoas abscess caused by Streptococcus intermedius and Sphingomonas paucimobilis. Magnetic resonance imaging showed multiloculated rim-enhancing lesion in right psoas muscle and joint effusion with osteomyelitis around sacroiliac joint. Intravenous antibiotics were administered after ultrasonographically guided abscess aspiration. Surgical drainage was done and his fever and symptom gradually subsided. (J Korean Acad Rehab Med 2008; 32: 465-468)
