Impaired tissue perfusion is often observed in patients with acute heart failure. We tested whether low-dose nitroglycerin (NTG) improves microcirculatory perfusion in patients admitted for acute heart failure. In 20 acute heart failure patients, NTG was given as intravenous infusion at a fixed dose of 33 mu g/min. Using Sidestream Dark Field (SDF) imaging, sublingual microvascular perfusion was evaluated before (T0, average of two baseline measurements) and 15 min after initiation of NTG (T1). In a subgroup of seven patients, SDF measurements were repeated after NTG had been stopped for 20 min. Capillaries were defined as microvessels with a diameter of < 20 mu m. Perfused capillary density (PCD) was determined as the parameter of tissue perfusion. Values are expressed as median and interquartile range (P25; P75). The median age of the subjects was 60 (52; 73) years, and 65% were male. Patients were stable before starting NTG. Nitroglycerin decreased central venous pressure [17 (13; 19) mmHg at T0 vs. 16 (13; 17) mmHg at T1, P = 0.03] and pulmonary capillary wedge pressure [23 (18; 31) mmHg at T0 vs. 19 (16; 25) mmHg at T1, P = 0.03]. It increased PCD [10.7 (9.9; 12.5) mm mm(-2) at T0 vs. 12.4 (11.4; 13.6) mm mm(-2) at T1, P = 0.01]. After cessation of NTG, PCD returned to baseline values (P = 0.04). Low-dose NTG significantly reduces cardiac filling pressures and improves microvascular perfusion in patients admitted for acute heart failure.