The authors analyzed the surgical results of posterior C1-C2 fusion in 18 cases of atlantoaxial instability. Posterior C1-C2 wiring with bone fusion(11 cases), C1-C2 transarticular screw fixation(6 cases), and occipitocervical fusion(1 case) have been performed for unstable odontoid process fractures(10 cases), transverse ligament injury(4 cases), os odontoideum(3 cases), rheumatoid C1-2 instability(1 case). Follow-up examination was performed in all patientts after a mean postoperativeduration of 39.9 months. Follow-up x-rays showed successful stabilization in 17 cases(94%). Postoperative neurological evaluation showed improvement in 16 cases(89%), stabilization in 2 cases(11%) and there was no major operative complication.