Aim of research: To examine the attitudes of students and teachers on the inclusion of students with developmental disabilities into secondary schools, to identify the differences in attitudes between students and teachers, and to identify the differences in attitudes between medical-school and grammar-school students. Respondents and methods: A cross-sectional study has been conducted on 156 students and teachers at Čakovec High School. The research instrument was a questionnaire consisting of two parts comprising 30 questions in total. The first part of the questionnaire was self-created. The second part comprises “Attitudes on Inclusion Questionnaire – SINKL 2.0” (Subotić and Anđić, 2016), and it consists of 19 questions in total. Results: The results show that students and teachers mostly have positive attitudes on the inclusion of students with developmental disabilities into secondary schools. There is no significant difference in comprehending inclusion as a moral imperative, in problem perception, and in professional support. Teachers agree with partial inclusion in a lesser degree than students. Medical students have a more positive attitude on inclusion as a moral imperative in comparison to grammar-school students. Also, medical students have a more negative attitude on problems with inclusion in schools in comparison to grammar-school students. Conclusion: On the basis of obtained results, it can be concluded that students’ and teachers’ attitudes on the inclusion of students with developmental disabilities into secondary schools are mostly positive. However, the respondents also think that regular schools still are not meeting the needs of children with developmental disabilities. It also seems that education, a direct contact with children with disabilities, and quality professional support are an important factor in forming attitudes among students and teachers.