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Aims: Although hypokalaemia is common among patients with heart failure (HF), the prognostic significance of baseline hypokalaemia and hypokalaemia during follow-up in HF patients receiving a mineralocorticoid receptor antagonist (MRA) remains uncertain., Methods and Results: Results of the EMPHASIS-HF trial in patients (n = 2737) with HF and reduced EF with mild symptoms, randomized to eplerenone or placebo, were analysed with regard to the presence or occurrence of hypokalaemia (serum K + <4.0 mmol/L) and the risk of cardiovascular death or hospitalization for HF (primary endpoint). Median follow-up was 21 months. Baseline hypokalaemia and hypokalaemia during follow-up were common occurrences (19.6% and 40.6%, respectively). Hypokalaemia during follow-up was associated with worse outcomes in multivariable analyses [hazard ratio (HR) 1.26, 95% confidence interval (CI) 1.05-1.52, P = 0.01] without evidence of interaction with eplerenone. In contrast, baseline hypokalaemia was associated with outcomes in the placebo group (HR 1.37, 95% CI 1.05-1.79, P = 0.02) but not in the eplerenone group (HR 0.87, 95% CI 0.62-1.23, P = 0.44; P for interaction = 0.04). Concurrently, eplerenone was found to be more protective in patients with baseline hypokalaemia vs. patients without baseline hypokalaemia compared with placebo (HR 0.44, 95% 0.30-0.64, P < 0.0001 vs. 0.69, 95% CI 0.57-0.83, P = 0.0001; P for interaction = 0.04). In patients without baseline hypokalaemia, eplerenone use decreased the rate of hypokalaemia during follow-up (HR 0.69, 95% CI 0.59-0.80, P < 0.001). A potassium level >4.0 mmol/L at 1 month after randomization mediated 26.0% (0.6-51.4%) of the eplerenone treatment effect (P = 0.04)., Conclusion: In HF patients receiving optimal therapy but not treated with eplerenone, baseline hypokalaemia was associated with worse outcomes. Conversely, hypokalaemia amplified the treatment effect of eplerenone., (© 2016 The Authors. European Journal of Heart Failure © 2016 European Society of Cardiology.)