Current research gives some indications about how inclusive education affects students with special educational needs (SEN), but there are too many different results to draw unequivocal conclusions. It is therefore important to focus on specific factors that could influence outcomes of inclusive education. In this study, we investigated whether there are differences between students with SEN who are included alone and students with SEN who are included in a class with other SEN students. A distinction was made between students with behavioural, cognitive and other problems. Data on 1839 primary school students with SEN in a Dutch cohort-study were used. For the academic achievement of students with different types of SEN, it did not seem to matter whether they were the only student with SEN in their class, or whether they had a few or more classmates with SEN. For most measures of socio-emotional functioning, no differences were found. For some other measures, however, differences were found. Because the results were not unequivocal, it would be important to try to replicate these findings in future research. (Contains 10 tables and 3 notes.)