The purpose of this study was to investigate the validity of the Progress Test that was specially designed for measuring the growth of knowledge and clinical reasoning skills in a problem-based medical curriculum. Scores and subscores of students from the different categories of the Progress Test were compared with their scores on a Clinical Reasoning Tests. Both the Progress Test and the Clinical Reasoning Test revealed the same pattern of increasing scores over the years, and had a high intercorrelation. Further analyses revealed that the clinical sciences subscore in the progress test explained the variations in the clinical reasoning test scores. The knowledge of the behavioural sciences subscore made a small but independent contribution. The knowledge of the biomedical sciences subscore did not have this independent effect. These outcomes are discussed in this paper from the perspective of development of medical expertise research and theory. Some educational consequences are also discussed.