Background: Radiographic sacroiliitis is the hallmark of ankylosing spondylitis (AS), and detection of acute sacroiliitis is pivotal for early diagnosis of AS. Although radiographic sacroiliitis is a distinguishing feature of AS, sacroiliitis can be seen in a variety of other disease entities. Case presentation: We present an interesting case of sacroiliitis in a patient with Paget disease; the patient presented with inflammatory back pain which was treated with bisphosphonate. This case demonstrates comorbidity with Paget disease and possible ankylosing spondylitis. We also present a review of the literature for other cases of Paget involvement of the sacroiliac joint. Conclusions: In addition, we review radiographic changes to the sacroiliac joint in classical ankylosing spondylitis as well as other common diseases. We compare and contrast features of other diseases that mimic sacroiliitis on a pelvic radiograph including Paget disease, osteitis condensans ilii, diffuse idiopathic skeletal hyperostosis, infections and sarcoid sacroiliitis. There are some features in the pelvic radiographic findings which help distinguish among mimics, however, one must also rely heavily on extra-pelvic radiographic lesions. In addition to the clinical presentation, various nuances may incline a clinician to the correct diagnosis; rheumatologists should be familiar with the imaging differences among these diseases and classic spondylitis findings.