The aim of this study was to develop a valid and reproducible index of orthodontic treatment priority. After reviewing the available literature, it was felt that this could be best achieved by using two separate components to record firstly the dental health and functional indications for treatment, and secondly the aesthetic impairment caused by the malocclusion. A modification of the index used by the Swedish Dental Health Board was used to record the need for orthodontic treatment on dental health and functional grounds. This index was modified by defining five grades, with precise dividing lines between each grade. An illustrated 10-point scale was used to assess independently the aesthetic treatment need of the patients. This scale was constructed using dental photographs of 12-year-olds collected during a large multi-disciplinary survey. Six non-dental judges rated these photographs on a visual analogue scale, and at equal intervals along the judged range, representative photographs were chosen. To test the index in use, two sample populations were defined; a group of patients referred for treatment, and a random sample of 11-12-year-old schoolchildren. Both samples were examined using the index and satisfactory levels of intra- and inter-examiner agreement were obtained.