The Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial (ALLHAT) reported that primary cardiovascular outcomes were the same in hypertensive patients treated with thiazide-type diuretics as in those treated with calcium channel blockers or angiotensin-converting enzyme inhibitors. The aim of this study was to assess prescribing patterns of antihypertensive agents in a health maintenance organization (HMO) before and after the publications of ALLHAT results. Our analysis used computer-stored information from the pharmacy system of the HMO for the period between January 1, 1996, and December 31, 2003. The study assessed prescribing patterns for antihypertensive drugs in six classes: thiazide-type diuretics, other-type antihypertensive diuretics, angiotensin-converting enzyme inhibitors/angiotensin receptor blockers, calcium channel blockers, beta-blockers, and alpha-blockers. The monthly total number and relative percentage of dispensed prescriptions for each class of antihypertensive agent, including new prescriptions and refills, were calculated. The use of alpha-blockers for all prescriptions began to decrease after the closure of the alpha-blocker (doxazosin) arm of ALLHAT in January 2000. Prescriptions for the thiazide-type diuretics immediately increased after the ALLHAT publication in December 2002. During the first 6 months of 2003, the percentages of thiazide-type diuretics were statistically significantly higher compared with the predicted values. This pattern held for all as well as for new antihypertensive prescriptions. The ALLHAT publications had large effects on the antihypertensive prescribing patterns in our population. Prescription of thiazide-type diuretics significantly increased after the ALLHAT publication. Our findings establish that the response of physicians to new clinical evidence can be very rapid.