Background: Left ventricular (LV) remodeling is related to adverse outcomes in heart failure. The CorCap Cardiac Support Device (CSD; Acorn Cardiovascular, Inc, St. Paul, MN) is an implantable device that attenuates LV remodeling. Methods: The Acorn trial assessed the safety and efficacy of the CSD in 300 heart failure patients. Patients needing mitral surgery (n = 193) were randomized to mitral surgery alone or mitral surgery plus CSD. Patients who did not need mitral surgery (n = 107) were randomized to medical therapy or medical therapy plus CSD. The primary endpoint was a clinical composite based on changes in patient vital status, the need for major cardiac procedures for worsening heart failure, and a change in New York Heart Association (NYHA) class. Results: The proportional odds ratio for the primary endpoint favored treatment with the CSD (1.73 confidence interval [CI]: 1.07 to 2.79; p = 0.024). The CSD-treated patients received significantly (p = 0.01) fewer cardiac procedures indicative of worsening heart failure and had an improvement in New York Heart Association class (p = 0.049). There was no significant difference in survival between groups (p = 0.85). Treatment with the CSD led to a decrease in LV end-diastolic (p = 0.009) and end-systolic volumes (p = 0.017), an increase in the LV sphericity index (p = 0.026), an improvement in the Minnesota Living with Heart Failure score (p = 0.04), and the Short Form-36 Questionnaire (p = 0.015). There was no evidence for a significant difference (p = 0.43) in serious adverse events between the treatment and control groups. Conclusions: The results of the Acorn trial support the hypothesis that preventing LV remodeling with a CSD favorably impacts the untoward natural history of heart failure.