BACKGROUND: As lip augmentation becomes more popular, validated measures of lip fullness for quantification of outcomes are needed. OBJECTIVE: Develop a scale for rating lip fullness and establish its reliability and sensitivity for assessing clinically meaningful differences. METHODS: The initial Allergan Lip Fullness Scale (iLFS; a four-point photographic scale with verbal descriptions) was validated by eight physicians rating 55 live subjects during two rounds, conducted on one day. In addition, subjects performed self-evaluations. The revised Allergan Lip Fullness Scale (LFS), a five-point scale with a broader range of lip presentations, was validated by 21 clinicians in two online image rating sessions, ≥14 days apart, in which they used the LFS to rate overall, upper, and lower lip fullness of 144 3-dimensional (3D) images. Physician inter- and intra-rater agreement, subject intra-rater agreement (iLFS), and subject-physician agreement (iLFS) were evaluated. Additionally, during online rating session 1, raters ranked 38 pairs of 3D images, taken before and after lip augmentation, as "clinically different" or "not clinically different." The median LFS score difference for clinically different pairs was calculated to determine the clinically meaningful difference. RESULTS: Clinician inter- and intra-rater agreement for the iLFS and LFS was substantial to almost perfect. Subject self-assessments (iLFS) had substantial intra-rater reliability and a high level of agreement with physician assessments. Median LFS score differences for overall, upper, and lower lip fullness were 1 (mean: 0.63-0.69) for "clinically different" and 0 (mean: 0.28-0.36) for "not clinically different" image pairs; thus, clinical significance of a 1-point difference in LFS score was established. CONCLUSIONS: The LFS is a reliable instrument for physician classification of lip fullness. A 1-point score difference can detect clinically meaningful differences in lip fullness.