The coexistence of systemic lupus erythematosus (SLE) and ankylosing spondylitis (AS) is very rare, and, to the best of our knowledge, there are only 8 reported cases in the English literature. Here, we present another case with the coexistence of these two diseases, and review the clinical and laboratory features of the previously reported cases. A 55 year-old female patient, with a diagnosis of SLE with locomotor, skin, renal and hematopoietic system involvement, which had been confirmed by relevant autoantibody positivity, and hypocomplementemia and biopsyproven membranous lupus nephritis, was referred to our clinic suffered from typical inflammatory low-back pain after eight years of follow-up. Sacroiliac magnetic resonance imaging (MRI) confirmed the presence of bilateral active sacroiliitis with bone marrow oedema. HLA-B27 was positive and bilateral calcaneal spurs were also detected by conventional radiography. Therefore, the additional diagnosis of AS was made, eight years after the diagnosis of SLE. Inflammatory low-back pain typically responded to treatment with non-steroidal anti-inflammatory drugs. Including the present case, most of the reported cases of the coexistence of SLE and AS are female, and SLE generally precedes the occurrence of AS. The present case is also notable as the patient had both MRI confirmation of bilateral active sacroiliitis and HLA-B27 positivity. The coexistence of these two diseases with different genetic backgrounds in the same patient is much lower than expected based upon their prevalence in the general population. Although it has been suggested that the very rare combination of the susceptibility genes of each disease may explain the rarity of coexistence, epidemiological data concerning the genetic risks for the coexistence of SLE and AS are not available