Application of anonymous research data of the sdq within the preventive child healthcare: usefulness and pitfalls. For the early detection of psychosocial problems within the preventive child healthcare (pch), for children in primary and secondary school, the strengths and difficulties questionnaire (sdq) is used during assessments. The early detection with the sdq is facilitated by the use of cut-off points. These cut-off points are in general extracted from validation studies within an anonymous setting. This setting is different from the confidential, non-anonymous, setting of the pch in which the sdq questionnaire is reviewed by the health professional. There is, therefore, a discrepancy between the settings in which the sdq is validated and in which it is implemented. In the present study it is investigated whether this discrepancy creates a difference in the sdq regarding reliability, mean scores, and cutoff points. For this study questionnaires were used from confidential pch assessments (n = 6.594) and the anonymous youthmonitor (n = 4.613), both administered in the second year of secondary school. The reliability of the sdq was equal in both settings; the mean scores and cut-off points differed however significantly. It is, therefore, not valid to use the cut-off values for the sdq derived from anonymous settings within confidential settings (e.g. Pch) nor to directly compare the mean scores of these different settings. Keywords: preventive child healthcare, sdq, early detection, cut-off points, psychosocial behavior.