The evidence base for practicing medicine continues to improve. However, abundant data show that gaps exist between best evidence and practice. Moreover, health care costs are climbing at an alarming rate. We propose to ask three related questions: 1) how effective are computer decision-support systems for improving compliance with evidence-based guidelines and costs in the ambulatory setting; 2) what is the impact on guideline compliance of applications that allow clinicians to track and follow-up test results; and 3) what are the main barriers to acceptance of guidelines delivered via real-time clinical decision-support systems.