© 2015 Mayo Foundation for Medical Education and Research. Objective To assess the relationship between resting heart rate and long-term all-cause mortality in ambulatory patients with heart failure (HF) relative to age, considering that although heart rate has been strongly associated with mortality in HF, the influence of age on target heart rate is incompletely characterized. Patients and Methods Consecutive patients in sinus rhythm referred to an ambulatory HF clinic of a university hospital between August 1, 2001, and March 31, 2012, were included. Unadjusted and adjusted Cox regression analyses were performed to assess heart rate as a prognostic marker, both as a continuous variable and after categorization into quintiles. Smooth spline estimates and hazard ratios (HRs) were plotted for 2 age strata (<75 years vs ≥ 75 years) for each individual heart rate. Results A total of 1033 patients were included (766 men [74.2%]; mean age, 65.1±12.6 years). During a mean follow-up of 4.6±3.3 years (median, 3.8 years [25th-75th percentile, 1.9-6.9]), 476 patients (46.1%) died. Mortality was associated with a statistically greater heart rate in the total cohort (HR, 1.18; 95% CI, 1.11-1.26; P<.001). From a clinical viewpoint, this means an 18% increased risk for every 10-beats/min elevation in heart rate. The same characteristics were present in the relationship between heart rate assessed after 6 months and long-term mortality (HR, 1.30; 95% CI, 1.20-1.42; P<.001). Overall, the prognostic importance of heart rate in ambulatory patients with HF was largely influenced by patient age. Remarkably, in the elderly population (≥ 75 years), heart rate below 68 beats/min conferred an increased risk of death, whereas in younger patients, mortality exhibited a declining slope at even the lowest heart rates. Conclusion Our research, if applicable to the prospective management of patients with ambulatory HF, suggests that patients aged 75 years or older have the best outcomes with target heart rates of 68 beats/min; however, younger patients may benefit from lower heart rates, even below 55 beats/min.