The demands placed on an index for evaluating the need of orthodontic treatment are described. An index designed by the Swedish Medical Health Board is discussed against the background of the above mentioned demands. A new index for the need of orthodontic treatment is presented. It consists of a morphological part in which essential abnormalities are recorded and allotted points according to their severity. The index also has a functional part in which selected functional disorders of the masticatory system as well as occlusal disharmonies are recorded and allotted points. The reasons are given for the choice of the various features included in the index. Practical instructions for the use of the index are given. The new index for need of orthodontic treatment was tried under clinical conditions in two series of children, aged 6--16 years. The variation between the recordings by the same examiner on two occassions was studied by double determinations. With but few exceptions no systematic errors were found between the double determinations. The precision of the new index for need of treatment was found to be the same as of that given by the Swedish Medical Health Board. No systematic differences were found between recordings made by orthodontists and inexperienced examiners. The accidental error of the method was of the same order for orthodontists as for inexperienced examiners. Comparison of recordings made by four orthodontists showed systematic errors between the recordings of functional but not of morphological deviations. Ranking of the children according to the need of orthodontic treatment as judged with the new index differed from that obtained with the system of the Swedish Medical Health Board. The new index attached more importance to functional deviations. This was the main requirement in the construction of the new index.