BACKGROUND. The influence of an opinion leader intervention on adherence to Unstable Angina (UA) guidelines compared with a traditional quality improvement model was investigated. RESEARCH DESIGN. A group-randomized controlled trial with 2210 patients from 21 hospitals was designed. There were three intervention arms: (1) no intervention (NI); (2) a traditional Health Care Quality Improvement Program (HCQIP); and (3) a physician opinion leader in addition to the HCQIP model (OL). Quality indicators included: electrocardiogram within 20 minutes, antiplatelet therapy within 24 hours and at discharge, and heparin and ß-blockers during hospitalization. Hospitals could determine the specific indicators they wished to target. Potential cases of UA were identified from Medicare claims data. UA confirmation was determined by a clinical algorithm based on data abstracted from medical records. Data analyses included both hospital level analysis (analysis of variance) and patient level analysis (generalized linear models). RESULTS. The only statistically significant postintervention difference in percentage compliant was greater improvement for the OL group in the use of antiplatelet therapy at 24 hours in both hospital level (P = 0.01) and patient level analyses (P <0.05) compared with the HCQIP and NI groups. When analyses were confined to hospitals that targeted specific indicators, compared with the HCQIP hospitals, the OL hospitals showed significantly greater change in percentage compliant postinterventionin both antiplatelet therapy during the first 24 hours (20.2% vs. -3.9%, P = 0.02) and heparin (31.0% vs.9.1%, P = 0.05). CONCLUSIONS. The influence of physician opinion leaders was unequivocally positive for only one of five quality indicators. To maximize adherence to best practicesthrough physician opinion leaders, more research on how these physicians influence health care delivery in their organizations will be required. © 2003 Lippincolt Williams & Wilkins, Inc.