Sex estimation is a crucial step to outline the biological profile of unknown human remains. The frontal and maxillary sinuses have unique anatomical features and can be utilized efficiently in individual identification. Previous studies investigated the frontal and maxillary sinus indexes separately for sex prediction. However, comparing the predictive value of the frontal sinus index (FSI) to the maxillary sinus index (MSI) has not been investigated yet. This study aimed to evaluate the sex predicting value of FSI compared to MSI assessed on digital lateral cephalometry. This retrospective cross-sectional study included 88 digital lateral cephalometric images of known sex (44 males and 44 females) and age (20-50 years). Two radiologists measured the maximum anteroposterior sinus width and height of the frontal sinus and maxillary sinus. Then, the sinus index was calculated for each radiograph. The obtained frontal and maxillary sinuses parameters were tabulated, along with the patient’s sex and age, and subjected to statistical analysis. Our results revealed significant differences between males and females regarding frontal sinus width, FSI, and MSI. The discriminant function analysis obtained in this study accurately predicted sex with an overall accuracy of 71.6% (81.8% of males and 61.4% of females) using FSI, and 60.2% (63.6% in females and 56.8% in males) using MSI. Frontal and maxillary sinus indexes can be useful tools for sex prediction. Discriminant function equations were developed for both the frontal and maxillary sinus indexes with higher accuracy rates for FSI compared to MSI.