Previous investigators have suggested that patients suffering headache stemming from the C2-3 segment of the cervical spine can be identified by detecting an abnormal axis of rotation of that segment. The present study tested this hypothesis by correlating the location of the instantaneous axis of rotation (IAR) of the C2-3 segment with diagnostic blocks of the C2-3 zygapophysial joint in a sample of patients with headache. We found no significant correlation between the location of the axis and the response to diagnostic blocks. Previous false-positive assertions appear to be due to insufficient attention to the precision and reproducibility of the techniques used to determine IARs.