The volume of the air cavities in the paranasal sinuses is, even at present, not only the simplest but also the most important index employed in the evaluation of the paranasal sinus. We have measured the size of the paranasal cavities in 20 healthy Japanese subjects by reconstructing three-dimensional (3-D) computed tomography (CT) images. To avoid any bias in determining the cavity margin, we first measured bilateral maxillary sinuses of a macaque both by silicone injection and by 3-D reconstruction. Then the CT value for 3-D reconstruction was determined by comparison with the volume measured by direct injection. Each sinus of the healthy volunteers was then measured by 3-D reconstruction imaging. Paranasal sinuses were, individually and on the whole, apparently larger than previously reported: 90.1 ml in males and 72.5 ml in females, on average. Comparing with other body size indices, the increment of the size was quite apparent and the lower prevalence of sinusitis was assumed to play a role in this increased volume of paranasal sinuses in the modern Japanese population.