This study follows two groups of students (67 in all) through the 3 years of their high school biology education and examines the development of their systems thinking--specifically their models of the human body as a system. Both groups were composed of biology majors, but the students in one group also participated in a PBLbased extension program called "Medical Systems". Data was gathered by means of concept maps, which the students completed at four strategic stages of the learning process: beginning of 10th grade, end of 10th grade, end of 11th grade and end of 12th grade. At the end of the 3 year learning process, the students' showed more complex system models. They included a wider range of concepts in their maps, spanning hierarchy levels ranging from the molecular and cellular to the system level. We also found an increase in references to dynamic interactions, but this did not encourage the students to use cellular level processes when explaining phenomena that occur at the systems level. The impact of the PBL teaching method was strongly evident in the complexity of the Medical Systems program students' concept maps, which heavily emphasized "hierarchy" and "diseases" as system characteristics.