The purpose of this study is to assess the presence of sexual dimorphism in the foramen magnum size. We analyzed 211 human skulls from the collection of the Universidade Federal de Sao Paulo, with a record of sex and age determined using anteroposterior and transverse diameters of foramen magnum, and their differences by gender (p<0.05) were ascertained. Fischer linear discriminant function was calculated and the value for the classification of these variables was determined. All the dimensions were found to be higher, and in men's skulls, the foramen magnum size had low discriminating power and were accurately classified only in 66.5% skulls. Our results show that this quantitative indicator is of limited practical value and should be supplemented with qualitative indicators of sexual dimorphism in the occipital bone to improve the accuracy in the sex diagnosis.