Previous studies have demonstrated different tooth-width ratio discrepancies in different populations. The objective of this study was to determine maxillary to mandibular tooth-size ratios in a Peruvian sample; 200 children were selected who had complete permanent dentition, without clinically visible dental caries or proximal restorations, and no previous or active orthodontic treatment. Their dental casts were measured to the nearest 0.1 mm with a sliding caliper and a Vernier scale. The Kolmogorov-Smirnov test, t test, and Pearson correlation test were used. No significant differences were found in anterior (P =.713) and total (P =.174) tooth-size sums according to sex. The anterior tooth-width ratios were 77.78% +/- 2.44% for females and 78.39% +/- 2.81% for males (P =.103). The total ratios were 90.79% +/- 1.71% for females and 91.33% +/- 2.07% for males (P =.048). For this reason, the combined male and female anterior total ratio was calculated (78.09 +/- 2.64%). There were clinically significant tooth-size discrepancies in almost one third of the sample. The 2-standard deviation range from the Bolton standard did not predict clinically significant anterior and total tooth-width ratio discrepancies.