Because many of the signs and symptoms of acute decompensated heart failure (ADHF) are nonspecific (e.g., dyspnea), accurate diagnosis can be challenging. Highly specific indicators of ADHF can assist in early and accurate diagnosis, therefore providing a potential for better outcomes and cost efficiency. Demographic, clinical, laboratory, and electronically detected S3 data (Inovise Medical, Inc., Portland, OR) were collected in 340 emergency department patients with suspected ADHF. After hospital discharge, two blinded cardiologists determined whether ADHF was present. Total hospital charges were also recorded. The overall ED misdiagnosis rate was 14.0%, of which over 90% were a failure to recognize ADHF when it was present. The S3 was highly specific (94%) for ADHF and was valuable in combination with BNP values to improve the diagnostic accuracy in undifferentiated emergency department dyspenic patients. Misdiagnosed ADHF patients accrued over $2500 more in hospital charges than patients correctly diagnosed with ADHF, a 32% increase.