Purpose: To assess smoking, tooth brushing and oral cleanliness and their relationships among 15-year-olds in Tehran, Iran. Materials and Methods: A cross-sectional study based on World Health Organization criteria and the methods of the Second International Collaborative Study was carried out in autumn 2004 among 15-year-olds (n = 502) in Tehran. Data were based on a self-administered questionnaire and a clinical dental examination. Results: Smokers comprised 5% of the boys and 2% of the girls (p = 0.02). Smoking was more common among students of less-educated parents (50% vs. 30%, p 0.05). Of all students, 26% reported twice-daily tooth brushing; those of higher socio-economic backgrounds and girls did so more frequently. Of the smokers, 11% reported no tooth brushing compared to 6% of the non-smokers. Oral cleanliness was good for 13%, moderate for 32%, and poor for 55%; the rates associated positively with female gender (p = 0.002), having higher-educated parents (p = 0.03), and reporting a higher frequency of tooth brushing (p 0.001). Those students reporting twice-daily tooth brushing had less dental plaque and gingival bleeding (p = 0.01) on both anterior and posterior teeth. In multivariable analyses, the best predictors for a good level of oral cleanliness were female gender (OR = 2.0) or twice-daily tooth brushing (OR = 1.7). Conclusion: Oral cleanliness and tooth brushing among 15-year-olds were at poor levels, particularly among boys. Such poor levels call for intensive attempts to enhance rates of twice-daily tooth brushing and to improve its quality. For this age group, anti-smoking purposes should be combined into school-based oral health promotion programmes as well. Schlagwörter: adolescents, oral cleanliness, smoking, tooth brushing