The key result of the analysis is that special needs education (SNE) students differ from those who do not receive SNE. The SNE students tend to have higher delinquency levels. It was observed that the exclusion of SNE students had a relatively small impact on the estimation of overall prevalence levels of delinquency. In contrast, such exclusion had a major impact on incidence estimation. In addition, the prevalence of major social and economical risk factors of delinquency was higher in the population of students placed in SNE, clearly indicating that their inclusion was important for research focusing on the causal sources of delinquency. Much of self-report delinquency research is conducted in schools. In recent years, there has been a marked increase in the number of students who are placed in SNE groups within the school system. In this article, a review of the classical debate on the limits of what has been called the 'school criminology' was undertaken, followed by a description of the recent rise in SNE provision with special emphasis on Finland. Third, a large school-based but SNE-inclusive dataset was used to examine how the exclusion of SNE students would have influenced the prevalence and incidence estimates of delinquency. Fourth, the article assessed such effects as SNE exclusion would have on substantive research on the correlates of delinquency. The article concludes with a discussion of the problems and challenges involved when researchers have to keep up with the changes in the institutional infrastructure that they use for research. Figures, tables, references, and appendix