The relationship between cranial deformation and the occurrence of extra-sutural bones remains poorly understood. Scholars are divided about whether or not cranial deformation causes an increased occurrence of these bones, and even those who agree that deformation does have an influence often disagree about which locations on the crania are most impacted. Part of this issue clearly stems from the difficulties in finding appropriate samples from which to test the hypothesis that deformation leads to an increased expression of extra-sutural bones. The present study examines the effects of fronto-occipital and occipital-only deformation on the occurrence of extra-sutural bones through an analysis of 59 deformed and undeformed crania from the Philippines. Relative to the undeformed crania, both deformed samples showed an elevated number of extra-sutural bones. Using two different statistical tests, we identified a significantly higher occurrence of extra-sutural bones at two distinct cranial localities for both the fronto-occipitally deformed crania (coronal suture and asterion) and for those showing only occipital flattening when compared to the undeformed crania (lambdoidal and asterion). However, as our data reveal, individuals with highly elevated occurrences of extra-sutural bones are not necessarily deformed. As such, we argue that deformation practices may have a positive impact on the formation of extra-sutural bones; however, future studies using larger samples of deformed and undeformed crania—ideally from the same site—will be necessary to distinguish between the influence of cranial deformation on extra-sutural bone formation relative to other population factors which may play a role in shaping variation in this feature. Copyright © 2010 John Wiley & Sons, Ltd.