Acute heart failure (AHF) is a clinical syndrome with devastating prognosis. Despite considerable improvements in the treatment of chronic heart failure, most trials of new drugs for AHF, such as vasodilators, inotropes, and diuretics, have failed to show a prognostic benefit. Therefore, pharmacological treatment of AHF has changed very little, and loop diuretics have remained a cornerstone drug for decades. One of the emerging factors possibly playing an important role in AHF management is the time course of treatment. Several recent retrospective studies have highlighted the importance of early treatment in AHF; however, at the time, support from a prospective study with an adequate number of enrolled patients was lacking. The Registry Focused on Very Early Presentation and Treatment in Emergency Department of Acute Heart Failure (REALITY-AHF) was the first prospective study to specifically focus on the time course of treatment in the very acute phase and its prognostic implication in patients with AHF. Data from the REALITY-AHF revealed that early treatment with intravenous furosemide is significantly associated with lower in-hospital mortality. Although pathophysiological background of this association remains to be investigated, the time course of treatment may be a critical component of AHF treatment, and it will be important to take this factor into account in future clinical studies on AHF.