Psoas abscess in rheumatoid arthritis--an inperspicuous diagnosis.

Primary or secondary psoas abscess is infrequently seen but should not be forgotten. We report on two patients with RA who presented predominantly with back pain and who subsequently developed pyrexia. The correct diagnosis of psoas abscess was not immediately apparent. These cases and the discussion and comments which follow illustrate the difficulties that may be encountered and remind readers of this inperspicuous diagnosis.