Abstract: An improved protective mouthguard blank is adapted for in situ custom-fitting by an athlete/patient. A method for in situ custom-fitting is disclosed. The resulting in situ custom-fitted mouthpiece provides superior retention and protective characteristics. The protective mouthguard blank has a generally V-shaped channel corresponding to a normal dental arch and is formed from thermoplastic substances which have a softening, moldable temperature in the range of 120-160 degrees F. The protective mouthguard blank has outer side walls which are longer than the labial vestibule and the buccal vestivules of the athlete/patient. The resulting in situ custom-formed protective mouthguard consequently has an outer side wall which substantially entirely fills the athlete/patient's buccal vestibules and labial vestibule.