Background: The foramen magnum is an important anatomical opening in the base of the skull through which the posterior cranial fossa communicates with the vertebral canal. It is also related to a number of pathological conditions including Chiari malformations, various tumours, and occipital dysplasias. The aim of the study was to evaluate the morphology of the foramen magnum in adult individuals in relation to sex. Material and methods: The morphology of the foramen magnum was evaluated using 3D computer tomography images in 313 individuals (142 male, 171 female) aged 20-30 years.Results: The mean values of the foramen length (37.06 ± 3.07 vs. 35.47 ± 2.60 mm), breadth (32.98 ± 2.78 vs. 30.95 ± 2.71 mm) and area (877.40 ± 131.64 vs. 781.57 ± 93.74 mm2) were significantly higher in males than in females. A significant, positive correlation was found between foramen length and breadth. Significant correlations were reported for breadth and area of the foramen magnum and corresponding external cranial diameters in females. Round as well as longitudinal and horizontal oval-like types of the foramen shape were established according to the breadth/length index of the structure. All the cranial and foramen measurements were significantly higher in individuals with round-like type of the foramen magnum. Conclusions: There was a sexual dimorphism of the foramen magnum among the examined individuals. It was related mainly to its linear diameters and area, not to the shape. Unlike males, female skulls had higher correlation between the examined parameters of the foramen and proper external cranial measurements, which indicates more homogeneous growth in girls.