Main results of the Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial were published in December 2002. The Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure, published in May 2003, recommended thiazide-type diuretics as initial pharmacological treatment alone or in combination with another drug in most patients with hypertension. To assess changes from before to after these publications, we compared antihypertensive medication prescriptions filled by patients who initiated pharmacological antihypertensive treatment in a large managed care organization during 3 time periods: (1) July 1, 2001, to June 30, 2002 (before these publications; n=1354); (2) July 1, 2003, to June 30, 2004 (to assess short-term changes; n=1542); and (3) July 1, 2004, to June 30, 2005 (to assess extended changes; n=1865). The percentage of patients initiating antihypertensive treatment with a thiazide-type diuretic increased from 30.6% to 39.4% (P<0.001) between 2001-2002 and 2003-2004, and the increase was maintained at 36.5% in 2004-2005 (P<0.001 compared with 2001-2002 and P=0.33 compared with 2003-2004). Among patients without diabetes mellitus, renal disease, a history of myocardial infarction, or heart failure, the percentage initiating pharmacological antihypertensive treatment with a thiazide-type diuretic increased from 33.1% in 2001-2002 to 43.4% in 2003-2004 (P<0.001) and remained increased (41.0%) in 2004-2005 (P<0.001 and P=0.23 compared with 2001-2002 and 2003-2004, respectively). Despite a sustained increase in the use of thiazide-type diuretics, this study indicates that an opportunity exists to increase adherence to the Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure guidelines.