CASUS is an authoring system, which should enable physicians to produce problem-based computer learning programs with minimal technical effort and give them sound instructional support. The theoretical background of CASUS are constructivist approaches to learning and instruction, which deal mainly with the question, how to design problem-based learning environments. The paper presents the constructivist concept of CASUS and results of a forma-tive evaluation. Four authors were observed and interviewed while developing a learning case with CASUS. The evaluation pursued mainly the (...) question of which difficulties arise while producing a case with CASUS and which measures are necessary in order to improve the authoring system. Results show that with regard to technical aspects physicians can produce learning cases even without any special prior knowledge. There were far more problems with instructional aspects of the production of cases. Most of these difficulties were very specific to the content of the cases. Therefore, it seems that an appropriate way to design learning cases with high quality, is a cooperation between some authors with different expertise. (shrink)