Hospitalization for acute decompensated heart failure (ADHF) is an important outcome in clinical trials and heart failure registries; however, the optimal strategy to identify these hospitalizations using International Classification of Diseases, Ninth Revision (ICD-9) codes is uncertain. We sought to identify diagnostic codes that improve ascertainment of ADHF hospitalizations. Heart failure-related ICD-9 principal discharge codes were used to identify 2,202 hospitalizations within the Minneapolis Veterans Affairs Medical Center from 2009 to 2014. Two independent reviewers adjudicated 447 of these hospitalizations to determine the accuracy of each code. We then applied our findings to an unadjusted nationwide sample containing the same ICD-9 codes of interest, from which overall positive predictive value (PPV), sensitivity, and accuracy were calculated. Use of 428.x alone resulted in a PPV of 91.3% (95% confidence interval [CI] 91.0 to 91.7), sensitivity of 97.5% (95% CI 97.3 to 97.6), and accuracy of 89.7% (95% CI 89.4 to 90.0). Combining 428.x with 402.x1, 404.x1, 415, and 518.4 resulted in improved sensitivity (99.2%; 95% CI 99.0 to 99.3) and accuracy (90.7%; 95% CI 90.4 to 91.1) while maintaining a PPV of 91.1% (95% CI 90.7 to 91.4). Excluding chronic heart failure codes (428.22, 428.32, and 428.42) from the proposed strategy resulted in an improvement of PPV to 92.3% (95% CI 92.0 to 92.6), although sensitivity and accuracy decreased to 96.6% (95% CI 96.3 to 96.8) and 90.0% (95% CI 89.6 to 90.3), respectively. In conclusion, a combination of codes including 428.x, 402.x1, 404.x1, 415, and 518.4 improves sensitivity and overall accuracy in ascertaining ADHF events compared with 428.x alone. This strategy could be further improved by manual adjudication of chronic heart failure codes.