Our work explores an interactive open learner modelling (IOLM) approach where learner diagnosis is considered as an interactive process involving both a computer system and a learner that play symmetrical (to a certain extent) roles and construct together the learner model. The paper presents an application of IOLM for diagnosing and fostering a learner’s conceptual understanding in a terminological domain. Based on an experimental study, we discuss computational and educational benefits of IOLM in terms of improving the quality of the obtained learner model and fostering reflective thinking.