Background: In the Netherlands, as in many other countries, there are indications of an inclusive school policy for children with Down syndrome. However, there is a lack of studies that evaluate to what extent this policy has actually succeeded in supporting the mainstreaming of these students. Method: For the period 1984-2011, the number of children with Down syndrome entering regular education and the percentage of children still in regular education after 1-7 years were estimated on basis of samples from the database of the Dutch Down Syndrome Foundation. These estimations were combined with historical demographic data on the total number of children with Down syndrome in primary school age. Validity of the model was examined by comparison of the model-based estimations of numbers and percentages in regular education with relevant available empirical data from the Dutch Ministry of Education and from Dutch special schools. Results: The percentage of all children with Down syndrome in the age range 4-13 in regular primary education has risen from 1% or 2% (at the very most about 20 children) in 1986-1987, to 10% (about 140 children) in 1991-1992, to 25% (about 400) in 1996-1997, to 35% (about 650) in 2001-2002 and to 37% (about 800) since 2005-2006. The proportional increase stopped in recent years. Conclusion: During the 1980s and 1990s, clearly more and more children with Down syndrome were in regular education, being supported by the then existing ad hoc regulations aimed at providing extra support in regular education. In the Netherlands, in 2003, these temporary regulations were transformed into structural legislation for children with disabilities. With regard to the mainstreaming of students with Down syndrome, the 2003 legislation has consolidated the situation. However, as percentages in regular education stayed fairly constant after 2000, it has failed to boost the mainstreaming of children with Down syndrome. The results of this study are discussed in the context of national and international legislation and educational policy.