Critically ill patients commonly show a decrease in hemoglobin concentration during their stay in the intensive care unit. The purpose of the present study was to evaluate whether nonbleeding patients with acute coronary syndrome (ACS) show a similar decrease of hemoglobin, and thereby furnish reference values and analyze possible mechanisms. In this retrospective, descriptive study, the charts of all patients with ACS hospitalized between January 2004 and September 2005 were screened with regard to patient characteristics, time course of hemoglobin, as well as clinical parameters, concomitant drug therapy, and fluid balances. One hundred three nonbleeding patients with ACS were analyzed. They showed an average hemoglobin decrease of 1.27 ± 1.00 g/dl (p <0.001). The decrease in hemoglobin level was observed during the first 12 to 24 hours; thereafter the hemoglobin concentration remained stable. We found a correlation among decrease of hemoglobin, parameters of stress, such as hypertension (p = 0.019), tachycardia (p = 0.004), pain (p = 0.043), and white blood cells (p = 0.021), as well as the intravenous administration of nitroglycerin (p = 0.004). In conclusion, during the first 24 hours in the intensive care unit the hemoglobin concentration of nonbleeding patients with ACS regularly decreases at 1.27 ± 1.00 g/dl. Any further decrease in hemoglobin level beyond these values should entail early active search of the bleeding source. We hypothesize that this decrease is due to normalization of the previous stress-induced hemoconcentration and “internal hemodilution” by nitroglycerin.