Although variations in physicians' practice patterns and use of resources are well documented, the reasons for these variations are less well understood. The uncertainty inherent in patient care may be one explanation. Existing measures of intolerance to uncertainty, developed in contexts outside of patient care, fail to explain these variations. To address this limitation, the Physicians' Reactions to Uncertainty scale was developed. A questionnaire containing an initial pool of 61 items was mailed to a random sample of 700 physicians in North Carolina and Oregon, stratified by specialty. The items covered nine areas of physicians' reactions to uncertainty derived from interviews with physicians and a definition of the concept affective reactions to uncertainity in patient care. Factor analysis of the 428 responses received yielded two primary factors that accounted for 58% of the common variance among the 61 items. Items with unambiguous loadings on these factors defined two reliable and readily interpretable subscales: Stress from Uncertainty (Cronbach's alpha = 0.90, 13 items) and Reluctance to Disclose Uncertainty to Others (alpha = 0.75, 9 items). By virtue of its clarity and good psychometric properties, this new measure promises insights into the role that uncertainty plays in physicians' resource utilization and practice patterns.