Ankylosing spondylitis is a disease in which inflammation of joints, most often in the axial skeleton, can lead to reactive fibrosis and eventual joint fusion with associated immobility and kyphosis. The disease often involves extra-articular features, such as uveitis and aortic regurgitation, as well as associated inflammatory conditions of the intestines. Its etiology is unknown. Ankylosing spondylitis most commonly presents in young males (15-30 years old) as persistent low back pain and stiffness that is worse in the morning and at night and improves with activity. The authors report the case of a young male athlete whose symptoms were initially incorrectly diagnosed as sacroiliac joint instability and dysfunction and later as a sacroiliac stress fracture before further workup revealed a seronegative spondyloarthropathy and the diagnosis of ankylosing spondylitis. The patient was prescribed oral indomethacin daily by the attending rheumatologist and started on a slow progression of return to running, jumping, and weight lifting. Within 4 weeks of beginning this treatment, the patient had complete cessation of pain with the medication. At follow-up 1 year after graduation from his university, the patient was nearly symptom free and working in a non-heavy labor job. The purpose of this case report is to remind sports medicine physicians of the prevalence of rheumatologic diseases in general and ankylosing spondylitis in particular and of the various ways in which spondyloarthropathies may present in athletes. Increased suspicion may lead to earlier diagnosis and treatment, potentially reducing illness severity and duration and improving the performance of athletes with this condition.