Objectives. Despite the widespread belief that orthodontics improves psychological well-being and self-esteem, there is little objective evidence to support this (Kenealy et al., 1989a; Shaw, O'Brien, Richmond, & Brook, 1991). A 20 year follow-up study compared the dental and psychosocial status of individuals who received, or did not receive, orthodontics as teenagers. Design. A prospective longitudinal cohort design with four studies of the effect of orthodontic treatment. Secondary analysis of outcome data incorporated orthodontic need at baseline and treatment received in a 2 X 2 factorial design. Methods. A multidisciplinary research programme studied a cohort of 1,018, 11-12 year old participants in 1981. Extensive assessment of dental health and psychosocial well-being was conducted; facial and dental photographs and plaster casts of dentition were obtained and rated for attractiveness and pre-treatment need. No recommendations about orthodontic treatment were made, and an observational approach was adopted. At the third follow-up 337 (30-31 year olds) were re-examined in 2001. Results. Participants with a prior need for orthodontic treatment as children who obtained treatment demonstrated better tooth alignment and satisfaction. However when self-esteem at baseline was controlled for, orthodontics had little positive impact on psychological health and quality of life in adulthood. Lack of orthodontic treatment where there was a prior need did not lead to psychological difficulties in later life. Dental status alone was a weak predictor of self-esteem at outcome explaining 8% of the variance. Self-esteem in adulthood was more strongly predicted (65% of the variance) by psychological variables at outcome: perception of quality of life, life satisfaction, self-efficacy, depression, social anxiety, emotional health, and by self-perception of attractiveness. Conclusions. Longitudinal analysis revealed that the observed effect of orthodontic treatment on self esteem at outcome was accounted for by self esteem at baseline. Prior need for treatment assessed in childhood made a small contribution to the prediction of self-esteem 20 years later in adulthood. Dental status in adulthood, whilst statistically significant, appeared to be of minor importance in a model that included other psychological variables. When prior need for treatment was taken into account there was little objective evidence to support the assumption that orthodontics improves long-term psychological health. © 2007 The British Psychological Society.