The aim of this study was to produce a treatment difficulty index (TDI) for unerupted maxillary canines. Thirty treated cases, each with an unerupted unilateral maxillary canine, were graded by 14 consultant orthodontists in terms of perceived alignment difficulty, and the four main factors which had contributed to each grade were listed in order of importance. The relationship between the grade and the contributory factors was then examined using regression analysis, and weightings were derived. These were applied to each factor, in order to derive a difficulty score total for each case. Linear regression analysis of difficulty scores against consultant grades produced an R2 value of 54.7 per cent using the original equation and an R2 of 52.3 per cent using values rounded to the nearest half. Both indicated a moderate level of agreement between allocated difficulty grade and calculated difficulty scores. The index provides a useful treatment planning aid for the management of impacted maxillary canines.