The purpose of this study was to examine the reliability and validity of questionnaire estimates of sedentary time in adolescents. Adolescents (n=98) completed Youth Risk Behavior Survey (YRBS) sedentary items and the Adolescent Sedentary Activity Questionnaire (ASAQ) twice. Actigraph GT3X+ accelerometers were used to obtain an objective measure of sedentary time as the criterion measure. Results indicated acceptable levels of reliability for the YRBS TV time item (rho=.82), but a lower estimate for the computer/video game item (rho=.68). When examined for the entire week, reliability estimates for all categories of the ASAQ were acceptable (R.70), except for the education and travel categories. Validity evidence of the ability of the questionnaires to estimate accelerometer-determined sedentary time was poor (r .16; Spearmans rho.15). In conclusion, the YRBS sedentary items and the ASAQ demonstrated moderate to good evidence of reliability and poor evidence of validity for measuring sedentary time in adolescents.