BACKGROUND: Mineralocorticoid receptor antagonists (MRA) reduce morbidity and mortality in heart failure with reduced ejection fraction but can cause hyperkalemia and acute kidney injury. Guidelines recommend measurement of serum potassium (K) and creatinine (Cr) before and serially after MRA initiation, but the extent to which this occurs is unknown. METHODS AND RESULTS: Using electronic data from 3 health systems 2005 to 2008, we performed a retrospective review of laboratory monitoring among 490 patients hospitalized for heart failure with reduced ejection fraction who were subsequently initiated on MRA therapy. Median age at time of MRA initiation was 73 years, and 37.1% were women. Spironolactone accounted for 99.4% of MRA use. Initial ambulatory MRA dispensing occurred at hospital discharge in 70.0% of cases. In the 30 days before MRA initiation, 94.3% of patients had a K or Cr measurement. Preinitiation K was >5.0 mmol/L in 1.4% and Cr>2.5 mg/dL in 1.7%. In the 7 days after MRA initiation among patients who remained alive and out of the hospital, 46.5% had no evidence of K measurement; by 30 days, 13.6% remained untested. Patient factors explained a small portion of postinitiation K testing (c-statistic, 0.67). CONCLUSIONS: Although laboratory monitoring before MRA initiation for heart failure with reduced ejection fraction is common, laboratory monitoring after MRA initiation frequently does not meet guideline recommendations, even in patients at higher risk for complications. Quality improvement efforts that encourage the use of MRA should also include mechanisms to address recommended monitoring.