The accuracy of measuring the prevalence of delinquency by means of self-reported questionnaires is difficult to evaluate. This study assesses the differential validity of self-reported delinquency in adolescents and, more specifically, self-reported police contacts because of suspected misconduct. This study was conducted as part of the Rotterdam Youth Monitor, a youth health surveillance system. Self-report data of pupils (mainly 12-15 years old) in the first or third grade of secondary school in the school years 2007-8 and 2008-9 (n = 23,914) were merged with police data. Of the pupils registered as a suspect, 62 percent admitted to having been interrogated at the police station. However, there were differences between groups. Multivariate analysis showed that Moroccan pupils and first-grade pupils were more likely to give an invalid response. Pupils who were registered for theft, vandalism or assault were more likely to give a valid response, whereas pupils who were registered for an offence involving fireworks were more likely to give an invalid response. We conclude that using only self-reported data to measure delinquency in an ethnically diverse population results in substantial bias. It is advisable to use multiple sources to measure the prevalence of delinquency.