Background: Cardiovascular drugs play a major role in the pre- and postoperative care in neonates with congenital heart disease. Management strategies aim to optimise contractility, improve diastolic function, maintain adequate preload, and reduce afterload. Levosimendan, a novel inodilator agent, enhances myocardial contractility and causes peripheral and coronary vasodilation. Objectives: A systematic approach was used to evaluate the acute haemodynamic effects of levosimendan in critically ill infants with low cardiac output syndrome (LCOS). Methods: Infants received a continuous infusion of levosimendan, at a dose increased stepwise (range 0.1–0.2 µg/kg/min), during 48 h. Two near-infrared units were used to assess cerebral (frontal-parietal, c) and peripheral (thigh, p) perfusion and oxygenation. The changes in cerebral blood volume (ΔCBV), cerebral (cΔHbD) and peripheral (pΔHbD) intravascular oxygenation and the cerebral (cTOI) and peripheral (pTOI) tissue oxygenation index that followed levosimendan administration were continuously monitored. Blood pressure, heart rate, and temperature were continuously recorded. In addition, baseline and end-of-study pH, blood gases, lactate and haematocrit were determined. Results: Seven doses of levosimendan were investigated. The mean study time was 13.3 (7–19) h. Levosimendan produced an increase in cΔHbD (p < 0.05) and pΔHbD (NS) and a decrease in heart rate (p < 0.001) and lactate (p < 0.05). Trends showed an increase in mean blood pressure (NS). These results were independent of the effect of time. Mixed linear model analysis identified blood pressure changes and levosimendan as factors independently associated with cΔHbD. Conclusions: Levosimendan improves cerebral and systemic perfusion and oxygenation in critically ill infants suffering from LCOS.