An epidemiological investigation involving 765 Glasgow schoolchildren aged nine, twelve, and fifteen years was undertaken to assess the severity of malocclusion, the need for orthodontic treatment, and the proportion of children in each age group who had previously received treatment. An index termed the Malocclusion Severity Index (MSI) was developed to establish objectively the severity of malocclusion in each individual. Fifty sets of orthodontic study models and six orthodontists assessed the reproducibility and validity of the index. Although there was a significant reduction in the proportion of children in need of orthodontic treatment between 9 and 15 years of age, a considerable number were still in need of treatment; crowding of their dentitions being responsible for the majority of the treatment requirement. The MSI was found to be as precise and valid as previously developed occlusal indices, for estimating the treatment needs of sample populations.