Background Limited data exist for the role of serum potassium changes during hospitalization for acute decompensated heart failure (ADHF). The present study investigated the long-term prognostic value of potassium changes during hospitalization in patients admitted for ADHF. Methods Our study is a pooled individual patient data analysis assembled from 3 prospective cohorts comprising 754 patients hospitalized for ADHF. The endpoint was all-cause mortality within 180 days after discharge. Serum potassium levels and N-terminal pro-B-type natriuretic peptide (NT-proBNP) levels were measured at admission and at discharge. Results A percentage decrease >15% in serum potassium levels occurred in 96 (13%) patients, and an absolute decrease of >0.7 mmol/L in serum potassium levels occurred in 85 (12%) patients; and both were predictors of poor outcome independent of admission or discharge serum potassium. After the addition of other strong predictors of mortality-a 30% change in NT-proBNP during hospitalization, discharge levels of NT-proBNP, renal markers, and other relevant clinical variables-the multivariate hazard ratio of serum potassium percentage reduction of >15% remained an independent predictor of 180-day mortality (hazard ratio 2.06, 95% CI 1.14-3.73). Conclusions A percentage serum potassium decline of >15% is an independent predictor of 180-day all-cause mortality on top of baseline potassium levels, NT-proBNP levels, renal variables, and other relevant clinical variables. This suggest that patients hospitalized for ADHF with a decline of >15% in serum potassium levels are at risk and thus monitoring and regulating of serum potassium level during hospitalization are needed in these patients.