IMPORTANCE Facial muscle weakness from paralysis or muscle dystrophy can significantly affect lip strength and function. Lip muscle weakness may result in articulation difficulties and spillage of food, both of which are socially and functionally disruptive for patients. There are few quantitative data on the effect of facial paralysis on lip strength. OBJECTIVES To quantify the effect of facial paralysis and muscular dystrophy on lip strength and evaluate the effectiveness of targeted lip injection augmentation. DESIGN, SETTING, AND PARTICIPANTS Analysis of patients at the Johns Hopkins Hospital between January 1, 2008, and July 31, 2014, presenting for treatment of lip incompetence due to facial paralysis and facial muscular dystrophy was prospectively undertaken. Patients who had undergone direct surgical lip procedures were excluded. MAIN OUTCOMES AND MEASURES Lip pressure measurements, anterior bolus spillage, and articulation of bilabial sounds before and after treatment were assessed by a single speech pathologist. Lip pressures were measured with the Iowa Oral Performance Instrument. RESULTS Twenty-Two patients with unilateral facial paralysis were evaluated for this study. Three patients with facioscapulohumeral muscular dystrophy were also evaluated. In unilateral facial paralysis, central lip strength was reduced in all patients compared with sex-corrected normative data (mean [SD] central lip strength, 5.5 [2.5] kPa in females and 9.6 [4.6] kPa in males). Compared with the nonparalyzed side, labial strength on the paralyzed sided was reduced by 69%. After injection augmentation of the paralyzed side, labial strength improved across the entire lip. Mean lip strength improved by 0.7-fold in the central lip from 5.60 to 9.30 kPa (P = .009), by 1.4-fold on the paralyzed side from 2.2 to 5.33 kPa (P = .006), and by 0.4-fold on the unaffected side from 7.11 to 9.56 kPa (P = .12). Lip strength in the 3 patients with facioscapulohumeral muscular dystrophy were uniformly reduced across the entire lip and improved by 6-to 7-fold after injection augmentation. All patients were noted by the speech pathologist to have improved articulation of plosive sounds and decreased anterior bolus spillage after the injection. CONCLUSIONS AND RELEVANCE Labial strength is reduced across the lip in patients with unilateral facial paralysis. The Iowa Oral Performance Instrument is an effective tool for measuring labial strength and can be use to evaluate the effectiveness of facial reanimation procedures. Injection augmentation of the lip is a simple and effective means of improving labial strength, bilabial sounds, and anterior spillage in patients with facial paralysis or facial muscular dystrophy.