Abscess of the psoas muscle is an infrequent diagnosis at hospitals in Northern countries. We report on 16 patients who had this diagnosis during the period 1991-2001. Eight patients were immigrants who had previously been healthy and most of them had experienced symptoms for approximately 1 y. MRI or CT scans revealed spondylodiscitis in 6 of these patients and Mycobacterium tuberculosis was identified as the causative agent. With the exception of 1 patient who was exclusively treated with antituberculous agents, all 8 immigrant patients were successfully treated with antituberculous agents in addition to percutaneous drainage. The other 8 patients were Norwegians, 4 of whom had underlying conditions such as diabetes mellitus or drug abuse. The causative microorganisms were Staphylococcus aureus or Streptococcus spp., with the exception of M. tuberculosis in 1 case. The Norwegian patients had a more acute history of symptoms than the immigrant patients and 2 of them were in a septic condition on admittance. Two of the Norwegians died of serious infection; 5 were successfully treated with percutaneous drainage in addition to antibiotics and 1 was treated exclusively with antibiotic agents. The clinical history and microorganism associated with psoas abscess seemed to depend on whether or not the patient was an immigrant. Owing to increasing immigration, diagnosis of psoas abscess should be taken into account in Northern countries.