Background: While an extensive body of research has examined the outcomes of inclusion for pupils with special needs, in particular learning disabilities, its effects on the development of children with intellectual disabilities (ID) have been less explored. As inclusive practices tend to be more common for this group of children, it is important to acquire more knowledge on this issue. Methods: A comparative study with an experimental group of 34 children with ID fully included in general education classrooms with support, and a control group of 34 comparable children in special schools has been conducted. The progress accomplished by these two groups in their academic achievement and adaptive behaviour has been compared over two school years. Results: Included children made slightly more progress in literacy skills than children attending special schools. No differences were found between the progress of the two groups in mathematics and adaptive behaviour. Conclusions: Inclusive education is an appropriate educational option for primary pupils with ID who require extensive support in school.