BACKGROUND-: In the Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial (ALLHAT), a randomized, double-blind, practice-based, active-control, comparative effectiveness trial in high-risk hypertensive participants, risk of new-onset heart failure (HF) was higher in the amlodipine (2.5-10 mg/d) and lisinopril (10-40 mg/d) arms compared with the chlorthalidone (12.5-25 mg/d) arm. Similar to other studies, mortality rates following new-onset HF were very high (≥ 50% at 5 years), and were similar across randomized treatment arms. After the randomized phase of the trial ended in 2002, outcomes were determined from administrative databases. METHODS AND RESULTS-: With the use of national databases, posttrial follow-up mortality through 2006 was obtained on participants who developed new-onset HF during the randomized (in-trial) phase of ALLHAT. Mean follow-up for the entire period was 8.9 years. Of 1761 participants with incident HF in-trial, 1348 died. Post-HF all-cause mortality was similar across treatment groups, with adjusted hazard ratios (95% confidence intervals) of 0.95 (0.81-1.12) and 1.05 (0.89-1.25), respectively, for amlodipine and lisinopril compared with chlorthalidone, and 10-year adjusted rates of 86%, 87%, and 83%, respectively. All-cause mortality rates were also similar among those with reduced ejection fractions (84%) and preserved ejection fractions (81%), with no significant differences by randomized treatment arm. CONCLUSIONS-: Once HF develops, risk of death is high and consistent across randomized treatment groups. Measures to prevent the development of HF, especially blood pressure control, must be a priority if mortality associated with the development of HF is to be addressed. CLINICAL TRIAL REGISTRATION-: http://www.clinicaltrials.gov. Unique identifier: NCT00000542. © 2011 American Heart Association, Inc.