Vertical root fractures in maxillary first molars affect the mesiobuccal root in most cases. Among intrinsic factors directly related to increasing susceptibility to vertical root fractures, the thickness of the cement/dentin walls is a factor over which the clinician has a direct influence during the chemomechanical preparation. In a cross-sectional in vitro study, the mesiobuccal roots of fifty extracted human maxillary first molars were sectioned horizontally at 1, 3 and 5 mm from the apex. Cement/ dentin thickness was measured in the resulting 300 surfaces by using optical microscopy to an accuracy of X20 magnification. The obtained data were summarized and values were assessed statistically by ANOVA and post-hoc Tukey's range test. Buccal and lingual walls had the greatest thicknesses, whereas mesial and distal were variable and thinnest at the 3 mm level. The buccal wall had statistically significant differences in all surfaces (p<0.05). The 3 mm apical can be considered as a "danger zone" for instrumentation due to the variability of the thinner walls of the canal. The great variability of the buccal wall and the thinness of the proximal walls may explain the frequent buccolingual direction of vertical mesiobuccal root fractures.