OBJECTIVES: To analyze the changes in utilization and prescribing patterns of antihypertensive drugs before and after the publication of the ALLHAT results in 2002 that recommended the use of thiazide diuretics in new starts. METHODS: Utilizing pharmacy claims, member and provider data from a managed care plan of over 2 million members, this study selected two cohorts of patients who received two or more claims for antihypertensive or diuretic products from the same providers in the first 9 months of 2002 (Period 1) or the first 9 months of 2003 (Period 2). The providers who prescribed antihypertensive or diuretic medications for both periods were included. The patients were continuously enrolled adults who did not receive any antihypertensive or diuretic products in the 3 months prior to index date. Changes in physician prescribing patterns for initiation of hypertension treatment in Period 1 and Period 2 were analyzed. Utilization of different medications between the two periods, especially the likelihood of receiving thiazide diuretics, was also examined. RESULTS: The study identified 7,605 physicians who prescribed antihypertensive drugs to 25,519 patients in Period 1 and 26,300 patients in Period 2. Across the two periods, the percentage of physicians who prescribed any thiazide diuretics increased from 14.5% to 16.1% (p <0.01), while utilization of ACE inhibitors or CCBs as initial treatment decreased approximately 2% (p <0.01). A logistic regression model indicated that patients in Period 2 were 22.8% more likely to receive thiazide diuretics and 9.9% more likely to receive any diuretics than patients in Period 1 (p <0.01), controlling for demographics, comorbidities measured by chronic disease scores (CDS), and provider specialties. CONCLUSION: ALLHAT results increased prescribing of thiazide diuretics as initial treatment of hypertension.