We describe an HLA-B27 positive patient in whom posttraumatic pyogenic sacroiliitis led to complete unilateral sacroiliac joint ankylosis in the absence of any signs indicative of HLA-B27 associated spondyloarthropathy. Sacroiliitis is the pathologic hallmark - and usually one of the earliest pathologic manifestations - of ankylosing spondylitis (AS). Bilateral sacroiliitis is typical for ankylosing spondylitis. The frequency of asymmetric sacroiliitis may be higher in other inflammatory disorders, e.g., reactive arthritis, Reiters syndrome, spondylitis associated with psoriasis, or inflammatory bowel disease. Most but not all of these disorders show an increased prevalence among individuals who have inherited the HLA-B27 gene. In the context of this case, we discuss the differential diagnosis of unilateral sacroiliitis.