Purpose: The objective of the present study was to compare the self-assessed and the clinically determined findings of oral health and their determinants 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 among the 15-year-olds (N = 509). The data were based on a self-administered questionnaire and a clinical dental examination. Results: Altogether 78% of the 15-year-olds assessed their oral health as good or better. Based on self-assessment, 46% reported gingival bleeding, 28% reported the need for a filling and 23% for teeth straightening. Clinical examinations showed that 40% of the students had sound dentition (decayed, missing or filled teeth, DMFT = 0), 40% had current caries (decayed teeth, DT > 0), 92% had gingival bleeding and 26% had a definite need for orthodontic treatment. Most of those with DMFT = 0 and DT = 0 assessed their oral health as good or better. Compared with clinical findings, sensitivity for self-assessed need for fillings, gingival bleeding and the need for teeth straightening were 42%, 49% and 37%, respectively, whereas the corresponding specificities were 82%, 80% and 81%, respectively. Good or better selfperceived oral health was more likely among those with sound dentition (odds ratio, OR = 2.1, P = 0.01), with no selfassessed need for fillings (OR = 2.1, P = 0.01), with a self-assessed absence of gingival bleeding (OR = 2.9, P 0.001) or with highly educated parents (OR = 1.2, P = 0.007). Conclusions: When compared with clinical evaluations, the students most accurately detected healthy conditions. Educating the students on the signs of dental diseases could increase the reliability of self-assessment to provide a useful method for reporting oral conditions, especially in countries with developing oral health care systems. Keywords: adolescents, self-assessment, sensitivity, specificity, validity