Abstract Lesson planning imposes a significant burden on teachers as they need to prepare different lesson plans for different classes according to various constraints. SmartLP, a case‐based lesson planning system, has been implemented as a means of assisting teachers in constructing quality lesson plans more quickly. SmartLP enables teachers to retrieve previous lesson plans and customise them according to their constraints as they should be tailored to accommodate differences in students, teachers and the facilities available. An evaluation study was designed to examine the effectiveness of a Case Based Reasoning (CBR) approach through its cycle of retrieve, reuse, revise and retain in assisting teachers in constructing lesson plans and to assess the effects, in terms of time and quality, the system had on lesson plan preparation. This paper presents the results of the study. The study revealed that all the lesson plans constructed with SmartLP took significantly less time than lesson plans constructed without. In addition, where the planning tasks had greater similarities to the cases in the case base, they took even less time to be constructed. No significant difference in the quality of the plans, measured by a scoring system, was noticed between the control and experimental groups.