The amount of upper incisor display (UID) during smile and conversation is one of the most decisive components in aesthetic judgment. The purpose of this study was to find which soft tissue and underlying hard tissue factors contributed to the amount of UID during posed smile (PS) and at rest posture (RP). The subjects consisted of 76 young adults (33 men and 43 women; mean [SD] age, 24.79 [2.29] y) with skeletal and dental class I relationship, normal overbite/overjet, and minor crowding (<2 mm). After checking reproducibility of the amounts of UID during PS and at RP in lateral cephalograms with facial photographs, 15 hard and soft tissue variables were measured, and statistical analysis was done. There was no significant sex difference in the amount of UID during PS and at RP. The amount of UID during PS significantly increased when anterior maxillary height was longer, lower gonial angle was larger, occlusal plane to sella-to-nasion plane angle was steeper, interlabial gap at RP was larger, upper lip length at RP was shorter and upper lip elevation during PS was larger. Multiple linear regression analysis to predict the degree of UID during PS generated a 4-variable model (adjusted R = 0.607): upper lip elevation (the dynamic soft tissue variable), interlabial gap and upper lip length (the static soft tissue ones), and anterior maxillary height (the hard tissue one). The diverse causes of inappropriate UID according to the soft and hard tissue factors need different treatment approaches such as orthodontic treatment, periodontal treatment, orthognathic surgery, botulinum toxin, or myectomy. The clinician can use these variables as a guideline for differential diagnosis of inappropriate UID.