This paper reports on a study that examines physicians' understanding of concepts and decision making in problems pertaining to hypercholesterolemia and coronary heart disease (CHD). The study was carried out in two phases: (1) a simulated clinical interview in which two clinical problems were presented and (2) a session in which subjects responded to a series of questions. The questions were related to the analysis of risk factors, diagnostic criteria (DC) for determining elevated lipid values, and differential diagnosis for lipid disorders. The subjects included 12 family practitioners who were randomly selected from a continuing medical education program at McGill University. The results indicate that all subjects exhibited gaps in their understanding of domain concepts. In particular, most physicians demonstrated a lack of knowledge concerning the primary genetic disorders that contribute to CHD, as well as deficiencies in understanding the secondary causes of hypercholesterolemia. The majority of subjects tended to overestimate the lipid value intervals for determining patients at high risk. Physicians had no difficulty diagnosing the first patient problem of familial hypercholesterolemia, but failed to identify the problem of elevated lipids secondary to hypothyroidism. We observed a dissociation between subjects' conceptual understanding and their application of knowledge in solving patient problems. The implications of this work are discussed in terms of the cognitive dimensions of technologies for supporting learning and evidence-based decision making.