Several different factors have been hypothesized as explanations of variation in primate paranasal sinus size. Biomechanical forces, particularly those associated with mastication, are frequently evoked to account for differences in primate craniofacial pneumatization. To test whether masticatory stresses are responsible for maxillary sinus volume diversity, two platyrrhine species of the genus Cebus (C. apella and C. albifrons) were examined. The former has been identified as a hard object feeder, and many morphological differences between the two species are attributable to differences in the mechanical properties of their respective diets. Sinus volumes were derived from serial coronal CT scans of the crania of adults. Several external cranial measurements were used to scale sinus volume relative to the size of the face. Relative measures of maxillary sinus volume were compared using standard statistical techniques. In all comparisons, the two capuchin species do not differ from one another significantly at P < 0.05. Thus, this “natural experiment” fails to support the interpretation that biomechanical forces acting on the facial skeleton substantially affect the degree of paranasal pneumatization in primates. This result suggests that it is unlikely that the maxillary sinus performs any function in relation to masticatory stress; other factors must be responsible for the variation in sinus volume among primates. Anat Rec, 291:1414–1419, 2008. © 2008 Wiley-Liss, Inc.