Background: We examined diffusion of troponin testing in Medicare patients with unstable angina before the release of year 2000 American College of Cardiology/American Heart Association guidelines recommending measurement in all patients with acute coronary syndromes. Methods: We identified unstable angina admissions from Medicare administrative files for 22 Alabama hospitals over two time periods: 03/1997-02/1998 and 01/99-12/1999. Data were obtained from chart abstraction. Patients not confirmed for unstable angina by a clinical algorithm were eliminated. Results: For 1997-1998, 1116 (87.7%) of all identified cases were clinically confirmed for unstable angina, and 1176 (90.3%) were confirmed for 1999. In 1997-1998, 235 (21%) of unstable angina patients had troponin measured, compared to 822 (70%) in 1999. From 1999, patients with troponin measurement, vs. those without, more likely had typical angina (50.9%, 37.4%), chest pain on arrival (72.8%, 57.1%), and chest pain at rest (45.4%, 37.2%) and more often received EKG within 20 min of presentation (46.3%, 27.9%) (P<.0005 for all). Patients with abnormal troponin levels more often received angiotensin converting enzyme inhibitors (54.6%, 18.3%), cardiac catheterization (45.4%, 31.2%), and percutaneous coronary intervention (18.6%, 4.8%) (P<.05 for all). These associations remained significant after multivariable adjustment for patient and hospital characteristics. Conclusions: This study demonstrates increasing and appropriate use of troponin before guideline release. Our findings suggest that guidelines may codify currently accepted practice rather than always disseminate new knowledge. The same forces that lead to guideline development and release may also lead to changes in clinical practice before guideline release. © 2003 Elsevier Inc. All rights reserved.