The identity and taxonomic distribution of paranasal sinuses among living platyrrhines has remained a contentious issue (e.g., Cave  Am J Phys Anthropol 26:277–288 vs. Hershkovitz  Chicago: University of Chicago Press) largely because the ontogenetic data required for their detection and identification (e.g., Cave ; Maier  Cambridge, UK: Cambridge University Press, 99–132.) were not attainable without sacrificing valuable juvenile and subadult specimens. Non-invasive computed tomography (CT) scanning of ontogenetic series of skulls for 10 platyrrhine genera demonstrates the presence of maxillary and ethmoid sinuses, as well as homologs of the human sphenoid and frontal sinuses. Differences in the latter two sinuses between platyrrhines and hominoids highlight the need for early developmental data in establishing sinus homology. In particular, the identification of homologous recesses in the cartilaginous nasal capsule, from which sinuses later develop, emerges as the critical step. This developmental approach also reveals that the anterior and posterior ethmoid sinuses are each sets of serial homologs, a point which reconciles previous difficulties in establishing sinus homologies across mammalian orders (e.g., Paulli  Gegenbaurs Morphol Jahrb 28:147–178, 179–251, 483–564). © 2004 Wiley-Liss, Inc.