Study Design.In all, 4 cases of the coexistence of congenital spinal deformity (CSD) and ankylosing spondylitis (AS) were reviewed.Objective.To provide an insight into the unusual association between CSD and AS.Summary of Background Data.CSD combined with musculoskeletal anomalies, including clubfeet, Sprengel deformity, Klippel-Feil syndrome, and developmental dysplasia of the hip, has been described in the literature.Additionally, the rare coexistence between AS and other inflammatory joint diseases, such as systemic lupus erythematosus, mixed connective tissue disease, or Beh(c)et disease, has also been reported.However, to the authors' knowledge, the coexistence of CSD and AS has not been described in the literature.Methods.In all.4 patients presented with symptoms of lower back pain for at least 1 year.Among them.3 patients (cases 1, 2, and 3) were initially diagnosed as CSD, and classified as butterfly vertebra in 2 patients (case 1, case 3).incarcerated hemivertebra in 1 patient (case 2).Furthermore, a diagnosis of coexistence of CSD and AS was proposed in these 3 cases with a delay in the diagnosis of AS for 1 to 10 years.The fourth patient sought initial medical attention in our hospital without delay in diagnosis.In case 1, polysegmental Smith-Petersen osteotomy and transpedicular fixation were performed.and drugs were administrated.Case 2, case 3,and case 4 were treated with amethopterin, folic acid.salicylazosulfapyridine, and celecoxib.Results.At the latest follow-up, 4 patients had no back pain and were satisfied with the outcome.Conclusion.The coexistence of CSD and AS is very rare.However, it is necessary to consider the diagnosis of AS in CSD patients who present with symptoms such as inflammatory low back pain, morning stiffness, loss of global sagittal balance, and other clinical features can not be exclusively explained by CSD solely.