There is little information on the effectiveness of emergency department (ED) observation unit (OU) heart failure (HF) therapy. The authors' objective was to evaluate outcomes after implementation of an ED-OU treatment protocol for HF exacerbation. Unblinded assessment of the effectiveness of an HF protocol was performed, controlled by outcome for 9 months prior to implementation. This included diagnostic and therapeutic algorithms, cardiology consultation, close monitoring, patient education, and discharge planning. Adverse outcomes were defined as the 90-day rates of ED HF revisits, hospital HF readmissions, or death, as determined by chart review, computer database search, and phone follow-up. One hundred fifty-four patients were enrolled; 50 entered before, and 104 after protocol implementation. Only six (12%) in the preprotocol and one (1%) of the postprotocol group were lost to follow-up. After an OU visit, postprotocol 90-day ED HF revisit rates declined 56% (0.90-0.51; p<0.0000) and the 90-day HF rehospitalization rate decreased 64% (0.77-0.50; p=0.007). The 90-day rates of death and OU HF readmission decreased from 4% to 1% (p=0.096) and 18% to 11% (p=0.099), respectively. An intensive outpatient ED OU HF management protocol safely decreases 90-day rates of emergency department visits and inpatient hospitalizations.