With the purpose of establishing the normal figure of the distance between the odontoid tip and the Fischgold-Metzger's bimastoid line - a useful reference for the radiological diagnosis of basilar impression - the authors make the statistical study of the measurements on 127 craniograms of patients without occipito-cervical malformations and on 11 cases of basilar impression diagnosed clinical and radiologically. The correlations of that distance with age and with the distance from the odontoid tip to the McGregor's basal line are studied. The authors conclude that: 1) Above 10 years of age, the average distance from the odontoid tip to the bimastoid line is normally +0.56 ± 5.79 mm., the fiducial limits being -10.79 and +11.91 mm. Therefore, figures above 12 mm. must be considered as radiological index of basilar impression. 2) Correlation was observed between the distances from the odontoid tip to McGregor's and Fischgold-Metzger's lines, in 120 normal cases, above 10 years of age. 3) There was no difference between sexes in relation to the distance from the odontoid tip to the bimastoid line. 4) At least until 20 years of age there is correlation between this distance and age. Therefore, under that age, the diagnosis of basilar impression must be made in presence of lesser figures of this distance, the upper normal limit being around +8 mm. 5) In cases of basilar impression, the average distance from the odontoid tip to the bimastoid line was +21.09 ± 10.03 mm., significantly greater than the average of cases without occipito-cervical anomalies. 6) There was no correlation between the degree of basilar impression and the age and between the distances from the odontoid tip and McGregor's and Fischgold-Metzger's lines in the cases of basilar impression.