In the last decades, the inclusion of pupils with special educational needs (SEN) into regular education has been promoted world-wide. Research has shown that the opportunity for social participation is one of the main motives of parents for sending their children to a mainstream school. However, many people question if the desired social participation of pupils with SEN is realised in practice. Several studies show that positive contacts and friendships between children with special needs and their typical peers do not occur spontaneously. In addition, it has been shown repeatedly that teachers tend to overestimate the degree of social participation of pupils with SEN. They do not notice sufficiently if these pupils are being teased, ignored or rejected. Therefore, it is desirable to develop an instrument for teachers by which they can assess the social participation of pupils with SEN. A questionnaire might help teachers assessing the social participation of these pupils and possible problems in this area might be revealed. Subsequently, teachers may be prompted to foster the social participation of pupils with SEN, for example by encouraging contacts and relationships between these pupils and their typical classmates.On the basis of an exploration of literature (Koster, Nakken, Pijl & Van Houten, in press), a provisional description of social participation was formulated. Four key themes are central in this description:" interactions / contacts; " acceptance by classmates; " friendships / relationships; " perception of the pupil with special needs. Subsequently, on the basis of the literature exploration, a Questionnaire Social Participation has been composed. With this questionnaire, teachers can assess the degree of social participation of pupils. The four key themes are represented in the questionnaire. In this study, the first experiences with this questionnaire are addressed. The research question is as follows: 'Is the Questionnaire Social Participation a reliable and valid instrument to assess the social participation of pupils with SEN in regular primary schools?' Keywords: social participation, pupils with special educational needs (SEN), regular primary schools As mentioned above, in this paper the first experiences with the Questionnaire Social Participation will be described. In 75 Dutch school classes with one or more pupils with SEN (grades 1-3), the teachers fill in the questionnaire. Each teacher fills in the questionnaire for the pupil with SEN and for two typical classmates. The reliability of the total questionnaire and the reliability of the four key themes within the questionnaire will be examined. In addition, the construct validity will be examined by comparing the scores of the pupils with SEN on the questionnaire with the scores of the pupils without special needs. Based on international literature (e.g. Koster, Pijl, van Houten & Nakken, 2007; Monchy et al., 2004; Pijl, Frostad & Flem, in preparation; Scheepstra, et al. 1999) it is expected that, compared to the classmates, the social participation of the pupils with SEN will be assessed less positively.Secondly, the outcomes of the questionnaire will be compared with an external criterion. Since there is no other instrument which encompasses the total concept of social participation, we chose to use instruments which measure the key themes. For practical reasons, the study which will be described in this paper, is restricted to the key themes 'acceptance by classmates' and 'perception of the pupil with special needs'. A sociometric rating scale is used to measure acceptance. In grade 3, perception of the pupil with SEN is measured by the subscale 'social acceptance' of the Perceived Competence Scale for Children (Harter, 1982). In grade 1 and 2, the perception of pupils with SEN is measured by the social subscale of the Pictorial Scale of Perceived Competence and Social Acceptance for Young Children (Harter & Pike, 1984). It is expected that the degree of acceptance by classmates and the height of the social self-perception both correlate positively with the score on the Questionnaire Social Participation. At the time of writing this proposal, the data collecting is still taking place. Consequently, results are not yet available.Harter (1982). The Perceived Competence Scale for Children. Child Development, 53, 87-97. Harter, S. & Pike, R. (1984). The Pictorial Scale of Perceived Competence and Social Acceptance for Young Children. Child Development, 55, 1969-1982. Koster, M., Nakken, H., Pijl, S.J. & Houten, E.J. van (in press). Being part of the peer group: a literature study focussing on the social dimension of inclusion in education. International Journal of Inclusive Education. Koster, M., Pijl, S.J., Van Houten, E.J. & Nakken, H. (2007). The social position and development of pupils with SEN in mainstream Dutch primary schools. European Journal of Special Needs Education, 22, 1, 31-46. Monchy, M. de, Pijl, S.J. & Zandberg, T.J. (2004). Discrepancies in judging social inclusion and bullying of pupils with behaviour problems. European Journal of Special Needs Education, 19, 3, 317-330.Pijl, S.J. (2005) Interventies gericht op sociale integratie: training van sociale vaardigheden voor leerlingen met beperkingen in het regulier onderwijs, (Interventions which aim at social integration: social skills training for pupils with disabilities in mainstream education), in: B. F. van der Meulen, C. Vlaskamp & K.P. van den Bos (Eds.) 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The paper will be submitted to an international journal, which focuses on education of pupils with special needs, with the request of considering it for publication.