Previous studies concerned with the normal development of the sphenoid sinus have reported that the onset of initial pneumatization varies from 6 months to 4 years of life, and that the pneumatization is completed by 12 to 14 years of age. This study was designed to evaluate the age-related development of the sphenoid sinus pneumatization in children using magnetic resonance (MR) imaging technique. MR images of the sphenoid sinus in 152 children, ages 2 months to 163 months, were retrospectively reviewed. The degree of pneumatization was classified into four grades as 0, 1, 2, and 3 in axial images, and into five grades as 0, 1, 2, 3, and 4 in sagittal images. The numbers of grades were age-matched in the studied children. The pneumatized sphenoid sinus was identifiable as early as 2 months of age on axial image, and 4 months on sagittal image. Pneumatized sphenoid sinuses were identified in 100% of the subjects studied at the age of 3 years on axial image, and 6 years on sagittal image. All the children over 9 years old showed pneumatization of grade 2 or more both on axial and sagittal images. The results of this study may indicate that the sphenoid sinus begins and completes pneumatization earlier than previously described.