to establish the spatial relationship between the surfaces of foramen magnum (FM) and spinal cord (SC) identifying how much of the FM surface is occupied by the SC. Material and methods: 40 normal adults (23 females) with a mean age of 22 yrs (18-25) were evaluated through MRI to evaluate the sagittal and transversal diameter of foramen magnum and spinal cord. The surface area of FM and spinal cord was calculated, in the craniovertebral junction. Results: the sagittal diameter was consistently greater than the transversal diameter in the FM with a mean value of 34.78 mm. On the other hand, the transversal diameter was bigger in the spinal cord, with the mean value of 12.18 mm. The spinal cord occupies 82.93% of the FM surface in the craniovertebral junction. There was no difference between sexes. Conclusions: The better knowledge of craniovertebral junction anatomy is helpful in the neurosurgical planning and in the diagnosis of diseases that present intracranial hypertension.