OBJECTIVES: The ALLHAT study, published in December of 2002, indicated improved outcomes for hypertensive patients on thiazide diuretics. This study evaluated the utilization of thiazides in hypertensives in the year before and after the publication of these results in a national primary care database. METHODS: A retrospective review of an electronic medical record (EMR) database (GE Medical Systems Centricity) containing the ambulatory health record data for over 3.2 million individuals was conducted. Patients with an ICD-9 code (401.xx) for hypertension and a GPI code for any of the 5 classes of anti-hypertensives (excluding combinations): Beta blocker (BB), Ace inhibitor (ACE), Calcium channel blocker (CCB), Thiazide diuretics (THZD), Angiotensin receptor blockers (ARB), were identified for the years 2002 and 2003. Patients in each year on thiazides, on or after HPTN diagnosis, were taken as a percentage of those taking any of the 5 classes of anti-hypertensives. Quartile analysis was used to compare the rates of patients on THZD to all 5 classes of AA's for the four quarters of the year 2002 and 2003. RESULTS: Some 33,422 and 45,056 patients were identified for the years 2002 and 2003, respectively, with a diagnosis for hypertension and treatment for an AA. Of these 3,896 patients in 2002 and 7,023 patients in 2003 were on THZD, yielding a significant increase of 3.9% (p<0.001). Correspondingly, there was a significant decrease in the rate of patients on ACE (2.9%, p<0.001), BB (1.3%, p<0.001), CCB (0.5%, p<0.03), and a significant increase in the rate of patients on ARB (0.9%, p<0.001). CONCLUSION: A moderate but significant change in the prescribing behavior of physicians following the publication of ALLHAT guidelines was observed. Future research should employ a time series to confirm study findings.