BACKGROUND: The use of cost-sharing to control healthcare expenditures is increasing, but there is scant information about patients’ knowledge of cost-sharing or its influence on behavior. OBJECTIVE: The objective of this study was to evaluate what patients know about their individual levels of cost-sharing and how it influences decisions to seek care. STUDY DESIGN: We conducted a cross-sectional telephone survey with a 69% response rate. SUBJECTS: We studied a stratified random sample of 695 adult patients in an integrated delivery system: 266 subjects > or =65 years, 218 low-income subjects, and 211 subjects from the overall membership. MEASURES: We used perceived and actual levels of copayments for emergency department (ED) visits, office visits, and prescription drugs; and self-reports of copayment-related behavior changes. RESULTS: One third of subjects correctly reported their ED copayment, whereas three fourths correctly reported their prescription drug and office visit copayments. Over half of the subjects (57%) underestimated their ED copayment by $20 or more. Among patients who reported having any copayment, 11% described changing their behavior because of the copayment, ie, delayed or avoided emergency care. The perceived copayment level was strongly associated with behavior change (odds ratio, 3.9). Other significant factors included having more ED visits in the past 12 months and having a low health status. CONCLUSIONS: Patients have less knowledge of their ED cost-sharing levels than for other services. The perceived copayment amount was strongly associated with avoidance of or delays in emergency care. Further research is needed to determine whether these responses reflect greater efficiency or harmful decisions.