Introduction: Patients recently diagnosed with chronic heart failure (CHF) should be routinely monitored for hyperkalemia (HK) which may predict adverse clinical outcomes.Hypothesis: To examine the risk of adverse outcomes of hyperkalemia in CHF patients.Methods: All individuals in Northern Denmark with an incident record of CHF during 2000 to 2012, defined by hospital diagnosis, echocardiography, and ACEi and β-blocker treatment at 6 months after HF diagnosis were included in the study. The risk of K >5.0, was computed by Kaplan-Meier analysis. Reduced kidney function (RKF) was defined as eGFR <45 mL/min/1.73 m. Clinical outcomes were assessed 6 months pre/post HK in a before-after analysis, and were compared with matched CHF comparison patients without HK.Results: Among 12,221 patients with CHF, RKF was present in 5046 (41%), and was associated with higher age, female gender, and more comorbid conditions. Within a year after CHF diagnosis, 16.5% of the patients had experienced HK, which was associated with the degree of RKF (8.0% with normal kidney function, and 21.3%, 27.2%, and 32.9% in those with stage 3B, 4 or 5 RKF). In CHF patients with HK, the risk of any acute hospitalization 6 months before the HK event was 37.9%, increasing to 59.1% 6-months after the HK event [before-after relative risk, RR, 1.58 (1.51-1.65)]. Corresponding risks increased from 28.4% to 45.2% for any heart disease hospital diagnosis after HK (1.66 (1.57-1.76)), from 2.2% to 3.5% (1.82 (1.41-2.34)) for cardiac arrhythmias, and from 2.8% to 12.8% (4.95 (4.07-6.01)) for intensive care unit (ICU) admissions. Six-month mortality following HK was 23.2%, compared with 6.0% in the non-HK comparison cohort. Compared with matched CHF patients without HK, 6-months adjusted hazard ratios for clinical outcomes were increased in CHF patients with HK: 2.06-fold for acute hospitalization (3.79/1.85); 4.80-fold for ICU admission (6.22/1.30), and 4.39-fold for death.Conclusions: More than 40% of CHF patients had RKF, and the risk of developing HK was strongly associated with degree of RKF. The first HK event predicted severe clinical outcomes and death in CHF patients. These data support the need for regular K measurement to identify CHF patients at risk of serious outcomes.