This paper discusses the validity of self-report data. It appears that self-report data are not equally valid among all ethnic groups. Rather large differences are apparent in the tendency of boys with official police contacts to admit delinquent activities. Youngsters from Morocco and Turkey were much more reticent about admitting delinquent activities than those born in The Netherlands or coming from Surinam. These differences in willingness to admit delinquent behaviour are related to social control variables, the number of police contacts, and knowledge of the Dutch language. A problem for etiological research is reported: variables which are considered to cause delinquency are also related to the tendency to admit involvement in criminality. Overall, arrest data probably provide the best indicator for comparing criminal involvement between ethnic groups