Purpose: This study investigated the role of mothers in their preadolescents' oral self-care (OSC) and oral health in Tehran, Iran. Materials and Methods: In 2005, two self-administered questionnaires provided data on a random sample of 9-year-old children (N = 416) and their mothers. Brushing frequency, use of fluoridated dentifrice and the number of sugary snacks between meals served as OSC assessment criteria for both children and their mothers. Supervision of the child's tooth brushing and control of the child's sugary snacking indicated the mother's role that was defined as either active or inactive. Decayed, missing or filled teeth (DMFT + dmft) values facilitated the assessment of children's dental status in a voluntary clinical examination for children. The child's twice-daily tooth brushing, sound dentition and good oral hygiene were taken as the outcome measures for evaluating the impact of the mother's role. The chi-square test and logistic regression model were used for the analyses. Results: Half of the mothers were assessed as active in the supervision of their children's tooth brushing. Supervision was clearly more frequently reported by the mothers whose own OSC was favourable. In the logistic regression model, the strongest factors contributing to the explanation of twice-daily tooth brushing were the child's female gender (odds ratio, OR = 1.6; 95% confidence interval, 95% CI = 1.1 to 2.4) and the mother's active supervision (OR = 1.5; 95% CI = 1.0 to 2.2). Having sound dentition was most strongly explained by the mothers' active supervision of their children's tooth brushing (OR = 2.4; 95% CI = 1.3 to 4.5). Conclusions: The mothers' supporting role positively reflects on twice-daily tooth brushing and sound dentition in preadolescents. More preventive efforts through mothers should be considered during the planning of promotion programmes for preadolescents. Keywords: behaviour, mother-child relationship, oral health, supervision