Primary treatment of pectus carinatum (PC) is performed with an external brace that compresses the protrusion. Patients are 'prescribed' a brace tightening force. However, no visual guides exist to display this force magnitude. The purpose of this study was to determine the repeatability of patients in applying their prescribed force over time and to determine whether the protrusion stiffness influences the patient-applied forces and the protrusion correction rate. Twenty-one male participants (12-17years) with chondrogladiolar PC were recruited at the time of brace fitting. Participants were evaluated on three visits: fitting, one month postfitting, and two months postfitting. Differences between prescribed force and patient-applied force were evaluated. Relationships of patient-applied force and correction rate with protrusion stiffness were assessed. Majority of individuals followed for two months (75%) had a significantly different patient-applied force (p<0.05) from their prescribed force. Protrusion stiffness had a positive relationship with patient-applied force, but no relationship with correction rate. Patients did not follow their prescribed force. Magnitudes of these differences require further investigation to determine clinical significance. Patient-applied forces were influenced by protrusion stiffness, but correction rate was not. Other factors may influence these variables, such as patient compliance. Treatment Study - Level IV.