In brief: The arch lengths of the maxillary casts of 413 university football players were measured and compared with the arch lengths of 12 types of mouth-formed mouth guards to determine how closely each type of mouth guard conformed to NCM rules. The study found that, for a significant number of university football players, currently available mouth-formed mouth guards are not large enough to meet the NCAA rules and that a population of black athletes has a significantly larger mean arch length than a population of racially mixed athletes. The largest mouth-formed mouth guard met the NCAA rule for only 5.5% of football players of a predominantly black team.