Study Design. A case-control study of patients with isolated type II dens fractures treated with halo vest immobilization. Objectives. To evaluate age as a risk factor for failure of halo immobilization in patients with type II dens fractures. Summary of Background Data. The literature reports an average fusion rate of approximately 70% in patients with type II dens fractures treated by halo vest immobilization. Although many investigators have examined patent age as a risk factor for nonfusion using halo immobilization, all studies have been supported only by Class III data. These studies, consequently, carry little or no statistical significance. Therefore, a case-control study based on Class II data was designed to evaluate age as a risk factor for failure of halo vest immobilization in patients with type II dens fractures. Methods. Thirty-three patients with isolated type II dens fractures treated with halo vest immobilization at the University of Iowa Hospitals and clinics between 1983 and 1997 were included. Type II fractures were defined with plain radiography as per the Anderson- D'Alonzo classification. Cases were defined as nonfusions after halo immobilization, whereas control subjects represented successful bony unions attained with halo immobilization. Results. When the case and control groups were compared, there was no significant difference between the groups in the presence of concomitant medical conditions, sex; the amount of fracture displacement, the direction of fracture displacement, the length of hospital stay, or length of follow-up. Age more than 50 years was found to be a highly significant risk factor for failure of halo immobilization (P = 0.002; Fisher's exact test, two-tailed). The odds ratio of these data indicate that the risk of failure of halo immobilization is 21 times higher in patients aged 50 years or more. Conclusions. Surgical intervention should be considered in those patients aged 50 years or more who have a type II dens fracture, if it can be accomplished with acceptable risk of morbidity and death.