The present report is a detailed analysis of facial growth and oral function in a girl with juvenile rheumatoid arthritis of the temporomandibular joints. She was followed from 9 to 17 years of age prior to and after orthognathic surgery. Facial growth was assessed by facial photographs, dental casts, and roentgencephalometry, and oral function was assessed clinically by electromyography, kinesiography, and bite force. In addition, histological and histochemical analysis was performed on biopsy material from her masseter muscle obtained at the time of surgery. The study showed a clear relationship between facial growth and oral function. Dysplastic growth of the mandible led to an increasingly unstable occlusion with poor working conditions for the masticatory muscles. The muscles became weak, and even revealed marked structural histological and histochemical changes. Based on these observations it is suggested that the conventional treatment strategy with postponement of orthodontic or orthognathic surgical treatment until cessation of growth is abandoned and that early treatment should be undertaken to maintain occlusal stability throughout the growth period.