Blockade of venous drainage in the cavernous sinus, which may play a pivotal role in the pathophysiology of cluster headache (CH), could be triggered by local inflammation. It could also be favored by a constitutional narrowness of the cavernous sinus region. Before exploring the latter with magnetic resonance imaging (MRI), we determined whether external morphometric skull measures are different among CH patients (n = 25), healthy volunteers (n = 21), and migraine patients (n = 20). All subjects were males of comparable age distribution. Six measures were taken: inion-nasion perimeter, inion-nasion distance over the vertex; distance between the upper ends of tragus; diameter at the level of the temporal fossa; diameter at mid inion-nasion perimeter at ear level; and inion-nasion diameter. CH patients had significantly smaller values than healthy subjects and/or migraine patients in all but one measure (ANOVA and Duncan's post-hoc analysis). This may suggest that they have a narrower anterior/middle cranial fossa, and possibly a narrower cavernous sinus loggia, which needs to be confirmed by a quantitative MRI study.