Summary Background: The nasolabial angle, particularly its lower component, i.e. the upper lip inclination (ULI), is an important keystone in treatment planning. Normative data for this parameter are not available. Objectives: A quantitative evaluation of the aesthetic impact of ULI on perceived attractiveness and threshold values of desire for treatment was undertaken. Methods: The ULI of an idealized silhouette profile image was altered incrementally between 61 and 100 degrees. Images were rated on a Likert scale by pre-treatment orthognathic patients (n = 75), laypeople (n = 75), and clinicians (n = 35). Results: An ULI between 79 and 85 degrees is viewed as ideal, with a range of 73-88 degrees deemed acceptable. Angles above or below this range, down to 67 degrees and up to 94 degrees are perceived as slightly unattractive, and anything outside the range of 67-94 degrees is deemed very unattractive. For patients the threshold value of desire for treatment was 91 degrees and above and 64 degrees and below, and for both clinicians and lay people the threshold value was 94 degrees and above and 64 degrees and below. Patients appear to be more critical than lay and clinician groups. This stresses the importance of using patients as observers, as well as laypeople and clinicians, in facial attractiveness research. Limitations: The results are based on an idealized male Caucasian profile. Conclusions: It is recommended that in treatment planning, the range of normal variability of the ULI, in terms of observer acceptance, is taken into account as well as the threshold values of the desire for treatment.