Background: The most common cause of low back pain is degenerative disease of the intervertebral disc and other structures of the lumbar spine. However, in some cases other less frequent causes of such pain can be seen, for example septic facet joint arthritis. Until now, only 40 cases of such inflammatory changes within the spine have been reported in the literature. The disease isprobably underestimated due to improper diagnostic pathway. Case Report: The authors describe a case of a 53-year-old woman who was repeatedly hospitalized during a five-month period because of an acute, severe low back pain, with sphincter dysfunction, partially resembling sciatic symptoms. Physical examinations revealed also focal tenderness in the area of the lumbar spine. Inflammatory markers (ESR - erythrocyte sedimentation rate, CRP - C-reactive protein) were elevated. Conservative analgetic treatment brought only partial and temporary relief of the pain and symptoms. The final accurate diagnosis of isolated septic facet joint arthritis at the level of L5/S1 was established after several months from the onset of the first symptoms, after performing various imaging examinations, including bone scintigraphy as well as CT and MRI of the lumbosacral spine. The patient fully recovered after antibiotic therapy and surgery, which was proven in several follow-up examinations showing no relevant pathology of the lumbar spine. The authors broadly describe the etiology and clinical symptoms of the septic facet joint arthritis as well as the significant role of imaging methods, especially MRI, in diagnostic process. The authors also discuss currently available treatment options, both conservative and surgical. Conclusions: The diagnostic procedure of septic facet joint arthritis requires several steps to be taken. Establishing a correct diagnosis may be difficult, that is why it is important to remember about rare causes of low back pain and to perform detailed physical examination, laboratory tests and choose appropriate imaging techniques.