The article examines two important aspects of data quality in self-completion surveys of young people, taking advantage of a unique data source: Understanding Society: the United Kingdom Household Longitudinal Study. Young persons aged 10–15 are asked to complete a self-administered paper questionnaire at annual intervals. The number of completed interviews varies over waves from 4,049 to 5,020. Data are also collected from parents, providing important explanatory covariates for our analysis. Stronger parent–child relationship and higher mother’s involvement in education were associated with lower item nonresponse rate and lower inconsistency throughout waves.We also found some evidence for a negative panel conditioning effect with an increase of social desirability bias and measurement errors in the subsequent waves. There was a higher level of inconsistent responses and a higher probability of social desirability bias throughout waves in more sensitive items.