Based on an analysis of the concept of diagnosis, this paper tries to determine the interest of a «diagnostic» assessment of school competencies. Diagnosis can be regarded either as a report of facts or as an inference deducing the competency of pupil from his performance, like the illness can be deduced from the symptoms in medical diagnosis. The second way is the only one to be useful for a didactical purpose of diagnostic assessment. However, it means that the pupil is seen as an object to be analysed and it would not likely match with the educational intention to characterize him as a human subject.