Nitrite plays an eminent role in cardiovascular physiology and pathology, mediating hypoxic vasodilation, reducing ischemia–reperfusion injury, and regulating cardiac energetics and function. The role of circulating nitrite in critically ill patients has not been examined so far. To investigate whether whole blood nitrite can be determined reproducibly in an intensive care setting, 30 patients from a cardiology intensive care unit were enrolled in this study, no matter what the underlying disease. Blood was drawn from an arterial catheter and whole blood nitrite was determined, using a tri-iodide/ozone-based chemiluminescence assay after incubation with a ferricyanide-containing stabilization solution. Whole blood nitrite levels ranged from 35 to 1193 nmol/L (mean±SEM: 220±20 nmol/L). Myocardial infarction was associated with lower whole blood nitrite levels (200±53 nmol/L for elevated serum CK MB levels vs 432±95 nmol/L in the normal CK MB range, p=0.039). Neither impaired kidney function nor an inflammatory state was associated with higher or lower whole blood nitrite levels. In conclusion, whole blood nitrite can be measured easily and reproducibly in critically ill patients, regardless of renal function and inflammation. The origin of decreased nitrite levels in myocardial infarction is currently unclear and needs to be further elucidated.