Uncertainty is inherent in clinical medicine and may contribute to variability in physician practice patterns, patient satisfaction, and exchange of information. However, research on physician disclosure of uncertainty to patients is sparse. We measured the frequency of physician expressions of uncertainty to patients using audiotapes of visits to 43 physicians by 216 continuity patients in a university-affiliated general medicine clinic. We also analyzed the audiotapes using Roter Interaction Analysis. Physicians completed Gerrity's Physician's Reaction to Uncertainty scale and patients completed the Kranz Health Opinion Survey and a standardized satisfaction questionnaire. Physicians made verbal expressions of uncertainty in 71% of clinic visits. Physicians with greater self-rated reluctance to disclose uncertainty to patients made fewer expressions. Physicians who made more uncertainty expressions also used more positive talk and partnership building, and gave more information to patients. Physicians also expressed more uncertainty to patients with more education, greater desire for information, and more questions. Physician uncertainty expression were associated with greater patient satisfaction, but not independently of other physician verbal behaviors that were also associated with satisfaction.