Objective: Literature on participatory health care suggests that, though many patients desire basic information, a substantial number prefer a passive role. This variability is explored as a function of laypersons' beliefs about the nature of medical knowledge, referred to as epistemological beliefs, through the evaluation of a newly-developed instrument, the discipline-based epistemological beliefs scale-lay medical knowledge (DEBS-LMK). Design: Survey. Method: Items were developed to measure Schommer's hypothesized constructs of epistemological beliefs within a medical context: source, structure, and certainty of knowledge plus beliefs about acquiring knowledge - innate ability and speed of learning. Setting: Faculty, staff, and students ("n" = 367) from a Midwestern university provided complete responses to 63 medical epistemological beliefs items and the 18-item multidimensional locus of control scales (MHLOC). Results: Factor analysis confirmed the basic theoretical structure of the instrument. Scores on the DEBS-LMK were associated in predictable ways with MHLOC scales. Beliefs about doctors as omniscient authorities were associated with perceptions that powerful others are responsible for health status; in particular, beliefs that patients should be empowered to question and challenge their doctors predicted lower powerful others scores. Beliefs that medical knowledge depends on innate ability and quick learning predicted views that health status is due to chance factors, but beliefs in the need for quick learning were negatively associated with an internal health locus of control. Conclusions: Implications for research on information seeking and on interventions to improve patient/provider communication are discussed. (Contains 6 tables and 1 figure.)