Thirty-seven patients treated for os odontoideum are reviewed. In 20 patients the treatment was conservative and in 17 consisted of occipitocervical or atlanto-axial fusion. Two patients died after the operation and another, who lived abroad, was lost to follow-up. The remaining 34 patients were followed up for an average of eight years: 19 improved, 14 were unchanged and one deteriorated. In the subgroup of 25 patients without cord signs, there were no significant differences between the two modes of treatment. Analysis of the radiological features of the upper cervical spine in 21 patients revealed a minimal sagittal diameter of less than 13 millimetres to be associated with a high risk of permanent cord damage. There was no correlation with the degree of instability per se. On the basis of this analysis, guidelines are suggested for the management of patients with an os odontoideum.