This paper analyzes whether class size has an effect on the prevalence of mental health problems and well-being among adolescents in Swedish schools. We use cross-sectional data collected in year 2008 covering 2755 Swedish adolescents in ninth grade from 40 schools and 159 classes. We utilize different econometric approaches to address potential between- and within-school endogeneity including school-fixed effects and regression discontinuity approaches. Our results indicate no robust effects of class size on the prevalence of mental health problems and well-being, and we cannot reject the hypothesis that class size has no effect on mental health and well-being at all. (Contains 6 tables and 10 notes.)