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This study was proposed to evaluate the facial contour and electrophysiologic changes of the masseter and temporalis muscles before and after botulinum toxin A injection in the wide lower face (square face).The botulinum toxin A injections were performed on 10 patients for the treatment of square face with masseter hypertrophy. To obtain an objective evaluation of the change in the facial contour, physical measurements, cephalometry, and clinical photographs were taken; and for evaluation of the function of the masseter and temporalis muscles, electromyographic studies were performed before and 1, 3, 5, 7, 9, and 12 months after treatment.By physical measurements and cephalometry, the maximal reduction in lower facial contour (mean reduction, 6.6 mm by physical measurements and 7.5 mm by cephalometry) was observed 3 months after the injection, and increased slowly until 12 months after treatment. The maximal amplitude of the right and left masseter muscles decreased to the lowest value 1 month after treatment, with continuous increase being observed thereafter. There were statistically significant differences at all of the follow-up time points in reduction of lower facial contour by physical measurements and in electromyographic studies of the left masseter muscles. There was no hypertrophy of the temporalis muscle to compensate for the atrophy of the masseter muscles.In this study, there was a 2-month interval between the lowest value of the maximal amplitude of the surface electromyography and the maximal clinical effects following botulinum toxin A injection, and there was similarity between the recovery of the masseter function and the diminution of the clinical effect. The clinical effect of botulinum toxin A persisted for 12 months after treatment on physical measurements, and the authors felt that this long-lasting effect of botulinum toxin A beyond expectation could be explained by incomplete recovery of muscle function.