The size of the frontal sinus has known to be extremely variable among paranasal sinuses from complete agenesis to the abnormally large ones. Abnormally large frontal sinuses were reported by many authors as variable terms, such as pneumocele, hypersinus or pneumosinus dilatans, and the size of the frontal sinus was clinically important especially for surgeons. But in the past, the calculation of the size was limited only to the linear measurement. The authors have measured the area of normal frontal sinuses of 104 Korean adults in the routinely checked Caldwell's view with a planimeter. Bilateral frontal agenesis was 12 cases(11.5%), unilateral agenesis was 5 cases(4.8%) on the right side and 7 cases(6.7%) on the left side. Frontal sinuses larger than 3 S.D. over averages were regarded as abnormally large, and the size was 14.6cm² on the right side, and 19.3cm² on the left side.