Psoas muscle abscess is rare and presents a diagnostic challenge requiring a high index of suspicion. We report an unusual case of primary psoas abscess caused by Proteus mirabilis. Primary psoas abscess is most commonly present in children and is usually caused by Staphylococcus aureus. A total of 434 cases of psoas abscess has been reported. The subject is discussed noting the differences between primary and secondary cases, and emphasizing the importance of ultrasound and computerized tomography guided drainage of psoas abscess. A flow chart for the evaluation and management of primary and secondary abscess is presented, taking into consideration the success rate of the various methods of treatment described in the literature.