Nine surgically proven cases of pyogenic psoas abscesses are presented. The most common finding on computed tomography was diffuse enlargement of the psoas with a central low density area corresponding to an abscess cavity. This was noted in 7 patients. The anatomy of the psoas muscle and retrofascial area and the changing etiology and bacteriology of psoas abscesses are discussed. Escherichia coli, Staphylococcus, Klebsiella, Clostridium and Proteus were identified.