Spontaneous atlanto-axial (C1-C2) dislocation is an atlanto-axial instability, found in 10 to 30% of trisomy 21 patients, the majority of whom is asymptomatic. We report a case of a 21 years-old woman, with trisomy 21, admitted in our department presenting a spinal cord compression syndrome with right hemiparesis associated with a cervicalgia evolving for 3 months of admission without trauma. Standard cervical radiography showed a C1-C2 dislocation with posterior displacement of the odontoid process. A cervical computerized tomography revealed a C1-C2 dislocation with significant recoil of the odontoid process. A cervical magnetic resonance imaging (MRI) confirmed the bulbo-medullar junction compression on the dislocation. The surgical treatment consisted of a cervico-occipital fixation. The laxity of the transverse ligament is one of the main causes of C1-C2 dislocation; hypoplasia, malformation or complete absence of the odontoid process; are also predisposing factors. It must be early detected. The treatment of choice is surgical also by arthrodesis of C1 to C4 + graft and enlargement of the occipital foramen or occipito-cervical arthrodesis by synthetic graft and Cotrel-Dubousset system or occipito-C4 arthrodesis + laminectomy of C1 and enlargement of the occipital foramen.