BACKGROUND Botulinum toxin type A has been increasingly used to improve the lower face contour in masseter hypertrophy. Different dosages and injection techniques are suggested in the literature, but the ideal approach remains unclear. This study aimed to establish an objective masseter classification method for a personalized botulinum toxin type A injection protocol. METHODS A combination of clinical palpation, B-mode ultrasound examination, and anatomical dissection studies was applied to classify the masseter regarding bulging type on clenching and muscle thickness in a normal population and in patients desiring reduction of masseter hypertrophy. Based on these findings, a tailored botulinum toxin type A injection protocol was set up and evaluated in a prospective clinical study regarding masseter thickness, facial measurements, patient satisfaction, and complications. RESULTS A total of 504 masseters were classified into five bulging types (minimal, mono, double, triple, and excessive) and three degrees of thickness (<10 mm, 10 to 14 mm, and >14 mm). Two hundred twenty cases were treated using individualized botulinum toxin type A injection dosages (20 to 40 units) and sites (one to three per muscle); the treatment was applied in 220 cases, according to the respective classification. Masseter thickness decreased significantly, from 12.9±2.9 mm to 8.7±1.7 mm, 3 months after injection (p<0.01). The initial ratio of the widest width of the lower face to the intercanthal distance (3.3±0.18) was also significantly reduced (3.0±0.2; p<0.01). The overall patient satisfaction rate was 95.9 percent. No serious complications occurred.. CONCLUSIONS The present study provides the scientific basis for individualized botulinum toxin type A injection for masseter hypertrophy. This protocol allowed for reduction of injection dosage and complication rates, and showed beneficial clinical effects in terms of significantly reduced masseter volume and improved lower face contour. CLINICAL QUESTION/LEVEL OF EVIDENCE Therapeutic, IV.