The article discussed the issue of the diagnosis with the use of task-support-task procedure. A theoretical model of diagnosis based on the concepts by L. S. Vygotski, R. Case, and A. Bandura was described and developed. The model was tested on a group of non-disabled preschool children, and children with mild and moderate intellectual disability who were paired up accordingly to their mental age. Each pair was given a set of developmentally adapted tasks. The tool (44 tasks) was reliable and valid. The task-support-task procedure significantly affected the level of the task performance in all the children and allowed to define the scope of potential abilities, especially in the children with mild and moderate intellectual disabilities. Most of the task they did fell into the zone of proximal development.