To assess the meaning ascribed by patients to qualitative expressions of probability commonly used in medical care and to determine patient preferences for obtaining information when communicating with their physicians about medical risk numerically and/or qualitatively.Cross-sectional survey of consecutive patients.A university-based Department of Veterans Affairs medical center.133 patients sequentially seen in a general medicine clinic.Subjects were given a randomly ordered list of 12 common terms, each a qualitative expression of probability. They were asked to indicate what they understood to be the numerical meaning of each word. The patients' probability estimates were found to comprise two groups of five terms each, with high intercorrelations among the probabilities assigned to the terms in each group. Mean probabilities assigned to terms in the first group all were greater than 60%, and mean probabilities assigned to terms in the second group all were below 50%. When asked whether they wanted chance information to be provided in numerical or qualitative terms, 32% reported that they wanted it only numerically; 35.3% wanted it only qualitatively; 21.8% wanted the information either way; and 8.3% wanted the information both ways.The numerical meanings that patients ascribe to probability terms fall into identifiable patterns. While patients vary in the actual values they assign to terms, the relative meanings of terms show consistent trends.