Objective: The aim of the present paper is to study the indications for fluid bolus therapy (FBT) and its associated physiological changes in ED patients. Methods: Prospective observational study of FBT in a tertiary ED, we recorded indications, number, types and volumes, resuscitation goals and perceived success rates of FBT. Moreover, we studied key physiological variables before, 10 min, 1 h and 2 h after FBT. Results: We studied 500 FBT episodes (750 [500–1250] mL). Median age was 59 (36–76) years and 57% were male. Shock was deemed present in 135 (27%) patients, septic shock in 80 (16%), and cardiogenic shock in 30 (6%). Overall, 0.9% saline (84%) was the most common fluid and hypotension the most common indication (70%). ‘Avoidance of hospital/ICU admission’ was the goal perceived to have the greatest success rate (85%). However, although mean arterial pressure (MAP) increased (P < 0.01) and heart rate (HR) decreased (P = 0.04) at 10 min (P = 0.01), both returned to baseline at 1 and 2 h. In contrast, respiratory rate (RR) increased at 1 (P < 0.01) and 2 h (P = 0.03) and temperature decreased at 1 and 2 h (both P < 0.001). In patients with shock, 1 h after FBT, there was a median 3 mmHg increase in MAP (P = 0.01) but no change in HR (P = 0.44), while RR increased (P < 0.01) and temperature decreased (P = 0.01). Conclusions: In ED, FBT is used mostly in patients without shock. However, after an immediate haemodynamic effect, FBT is associated with absent or limited physiological changes at 1 or 2 h. Even in shocked patients, the changes in MAP at 1 or 2 h after FBT are small.