Unstable burst fractures of the atlas require sufficient immobilisation either with an external device or by surgical fusion. In patients with pre-existing congenital atlantal defects the decision whether to recommend non-operative or operative therapy is difficult. Treatment options are controversially discussed and standardised guidelines are not available. Although most surgeons advise operative fusion or osteosynthesis in atlanto-axial instability, non-operative immobilisation can be successful. We report on a patient with an unstable C1 burst fracture with a congenital posterior cleft in the atlantal arch who was treated with cervical extension and halovest immobilisation. Although callus bridging was initially observed, 9 months after the injury there was no sufficient fracture consolidation and surgical fusion had to be advised. While non-operative treatment may work in patients without congenital defects of the C1 arch, such is not recommended in patients who present with this anomaly.