Psoas muscle abscesses are a diagnostic and therapeutic challenge. Until recently, surgery was mandated for diagnosis and drainage of these deep posterior lesions. Scanning techniques such as computerized tomography, radionuclide imaging, and ultrasonography now enable noninvasive visualization of abnormalities of the psoas muscle. Patients with abscesses in the greater psoas muscle fall into two distinct groups. Six of 12 patients reviewed had no apparent predisposing conditions. These patients presented with subacute symptoms of fever, pain, and disability. Staphylococcus aureus was the predominant organism isolated. Psoas infections developed in six other patients secondarily to infection or trauma elsewhere in the abdomen. Gram-negative and enteric organisms were the predominant bacteria isolated from this group. Surgical drainage in selected patients and appropriate antimicrobial therapy is necessary for treatment of these infections. Late complications such as osteomyelitis are not unusual.