The shape of root canals cross-sectioned through their roots at 2 mm from their apices and its correlation with the D0 diameter of endodontic instruments was evaluated in 40 first upper molars. The molars were grouped according to age: under 13 yr (children), 18 to 20 yr (adolescents), 30 to 40 yr (adults), and over 50 yr. Evaluation of the root canal diameters revealed that the shapes were predominantly circular in the palatal canal, mostly flat in the mesiobuccal canal, and circular or flat in equal proportions in the distobuccal root. Age does not seem to affect the shape of the canals. Narrowing with age was statistically significant (p < 0.05) for palatal and mesiobuccal canals only. Correlation between the maximum diameter of the canals and the instruments was varied. Even in old age, diameters were observed that would require instruments of a size that would be impossible to use, because one internal diameter would exceed the root's external diameter in a different direction (i.e. intimal buccolingual diameter of #80 and external mesiodistal diameter of #70).