INTRODUCTION: Patients with symptomatic heart failure (HF) require additive therapies and have a poor prognosis. However, patient characteristics and clinical outcome between HF patients treated in the outpatient setting vs. those who are hospitalized remain scarce. METHODS AND RESULTS: The BIOlogy Study to TAilored Treatment in Chronic Heart Failure (BIOSTAT-CHF) included 2516 patients with symptoms and/or signs of HF: 1694 as inpatients and 822 as outpatients. Compared to ambulatory HF patients, inpatients had higher heart rate, urea, N-terminal pro-brain natriuretic peptide, lower blood pressure, lower estimated glomerular filtration rate, sodium, potassium, high-density lipoprotein cholesterol, had more often peripheral oedema, diabetes, anaemia, and were less often treated with beta-blockers and angiotensin-converting enzyme inhibitors (ACEi). Outpatients had a more frequent history of HF hospitalization and received more frequently beta-blockers and/or ACEi/angiotensin receptor blockers up-titrated to target doses (P < 0.001). Inpatients had higher rates of the primary outcome of death or HF hospitalization: incidence rate per 100 person-years of 33.4 [95% confidence interval (CI) 31.1-35.9] for inpatients vs. 18.5 (95% CI 16.4-21.0) for outpatients; adjusted hazard ratio 1.24 (95% CI 1.07-1.43). Subdividing patients into low, intermediate and high-risk categories, the primary outcome event rates were 14.3 (95% CI 12.3-16.7), 36.6 (95% CI 32.2-41.5), and 71.3 (95% CI 64.4-79.0) for inpatients vs. 8.4 (95% CI 6.6-10.6), 29.8 (95% CI 24.5-36.2), and 43.3 (95% CI 34.7-54.0) for outpatients, respectively. These findings were externally replicated. CONCLUSIONS: Marked differences were observed between inpatients and outpatients with HF. Overall, inpatients were sicker and had higher event rates. However, a substantial proportion of outpatients had similar or higher event rates compared to inpatients. These findings suggest that HF outpatients also have poor prognosis and may be the focus of future trials.