In this study Dutch subjects with four different levels of expertise (24 second-year, 24 fourth-year, 24 sixth-year medical students, and 24 internists with at least 4 years of experience) studied, diagnosed, and explained four clinical cases. Diagnostic accuracy increased with the increasing level of expertise. The number of concepts used and the number of detail concepts in the explanation protocols showed an inverted U-shaped relationship with level of expertise, whereas the experts' protocols matched better with canonical explanations of the cases. Constraining the processing time did not affect diagnostic accuracy but did affect the elaborateness of the explanations. It was concluded that advanced students applied more and better detailed biomedical knowledge than experts in clinical reasoning. Four figures, two appendixes (case study and chart of decisions) are included. (Contains 17 references.) (Author/SLD)