Assess patient knowledge of and response to drug cost sharing. Adult members of a large prepaid, integrated delivery system. Telephone interviews with 932 participants (72 percent response rate) who reported knowledge of the structures and amounts of their prescription drug cost sharing. Participants reported cost-related changes in their drug adherence, any financial burden, and other cost-coping behaviors. Actual cost sharing amounts came from administrative databases. Overall, 27 percent of patients knew all of their drug cost sharing structures and amounts. After adjustment for individual characteristics, additional patient cost sharing structures (tiers and caps), and higher copayment amounts were associated with reporting decreased adherence, financial burden, or other cost-coping behaviors. Patient knowledge of their drug benefits is limited, especially for more complex cost sharing structures. Patients also report a range of responses to greater cost sharing, including decreasing adherence.