Evidence on teachers' knowledge about somatic and mental chronic diseases among ID-adolescent compared to the knowledge parents and healthcare professionals have, is limited. The aim of this study is: (1) to assess the knowledge of teachers on the presence of chronic diseases in ID-adolescents; (2) to compare teachers with parents and healthcare professionals and parents with healthcare professionals regarding the knowledge on the presence of chronic diseases in ID-adolescents. We obtained data on 1044 ID-adolescents attending secondary schools, fully covering one region of the Netherlands. Teachers, parents and general practitioners (GPs) of the adolescents completed a questionnaire about the occurrence of chronic diseases in their child during the previous 12 months. The questionnaire was derived from the Dutch National Permanent Survey on Living Conditions questionnaire periodically administered in a representative population sample (n approximately equal to 10,000). Concordance between teachers, parents and healthcare professionals on the presence of chronic diseases in ID-adolescents was relatively low. In about half of all 66 dyads the concordance was for the most part fair and just in 10 dyads good to very good; nine of these latter cases concerned somatic chronic diseases. In addition, teachers reported mostly lower prevalence rates of chronic diseases in ID-adolescents compared to the parents, in particular on mental chronic diseases. Although prevalence rates of chronic diseases among ID-adolescents are very high, knowledge on this among teachers is limited. While information on chronic diseases in ID-adolescents is available among different informants, the disagreement between them reflects different points of view between the informants and probably indicates a lack of communication. The communication among teachers, parents and GPs should be improved to combine the knowledge and information on the presence of chronic diseases in ID-adolescents. This may provide opportunities to improve the support of these adolescents in their school career and in their transition from school to work. (Contains 3 tables.)