Background: Studies on the writing of students with intellectual disabilities have been scarce and unrepresentative. Therefore, the purpose of this research was to analyse this group of students' abilities to plan their texts. Method: A collective case study was carried out, with an eminently qualitative approach, although aided by an initial quantitative analysis. Fifteen students with unspecified intellectual disabilities were interviewed using open-ended questions. For data interpretation, content analysis and quantification of responses were used. Results: The results revealed the abilities and limitations of these students regarding the knowledge and use of one of the key processes of written expression (writing planning). Conclusion: The cognitive operations of planning, in which these students admitted the greatest problems, were the ordering and recording of ideas, and textual structuring.