To investigate the effects of example format (erroneous examples vs. correct examples) and feedback format (elaborated feedback vs. knowledge of results feedback) on medical students' diagnostic competence in the context of a web-based learning environment containing case-based worked examples, two studies with a 2 x 2 design were conducted in the laboratory. In the first study (domain: arterial hypertension; N = 153) erroneous examples were effective in combination with elaborated feedback with respect to strategic and conditional knowledge. In the second study (domain: hyperthyroidism, N = 124), elaborated feedback supported all aspects of diagnostic competence, especially conditional knowledge. (Contains 2 tables.)