© 2017 European Journal of Dentistry. Objective: The aim of this research project was to determine whether the sizes of the first molars allow clinicians to achieve the first goal of an ideal clinical outcome. Material and Methods: Seventy-eight sets of dental casts that have been presented to the American Board of Orthodontics were evaluated. A Boley gauge was used to measure the length from the mesiobuccal cusp to the distobuccal cusp of the maxillary first molar and the length from the mesiobuccal groove of the mandibular first molar to the occlusal embrasure between the mandibular first and second molars. These two measurements were taken on both sides of each set of dental casts for a total of four measurements per set. Results: The maxillary measurements ranged from 3.6 to 6.9 mm with an average of 5.2 mm. The mandibular measurements ranged from 5.0 to 8.0 mm with an average of 6.5 mm. The data were tested for normality and found to be equally distributed. A t-test revealed significant differences in tooth sizes between maxillary and mandibular first molars on both sides. On average, the mesiodistal length measured on maxillary first molars was about 80% of that of their mandibular counterparts. Only 5 of the 78 sets of dental casts evaluated had equal maxillary and mandibular measurements on one side (either left or right), and none of them had equal measurements on both sides. Conclusion: Clinicians have to understand that tooth size discrepancies do exist in patients and that these discrepancies make the completion of a perfect case challenging.