Previous research has shown that expert physicians' diagnostic performance improves when contextual information about a patient is available, while the performance of novices is not influenced by this source of information. These results are explained by supposing that experts' knowledge is organized in illness-scripts. This study examined this proposition. Subjects were 23 fourth-year and 22 sixth-year medical students, 23 interns in their second years of internship, and 22 family physicians. Students were from the Netherlands, and physicians were from the Netherlands or Belgium. Stimulus material consisted of 20 names of diseases. Subjects were asked to describe prototypical patients or to describe the clinical pictures involved in the diseases. Results are in agreement with the theory that with increasing expertise, knowledge structures qualitatively change toward more mature illness scripts. With increasing level of expertise, the proportion of patient characteristics in the subjects' descriptions of diseases increases, while the proportion of biomedical knowledge increases. Furthermore, experts tend to activate patient information without regard to probing condition, while intermediates activate this information only if explicitly asked. A 14-item list of references and two appendices with a list of the illnesses and examples of protocols subjects described are included. (SLD)