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In addition to the fluid intake in the form of intravenous maintenance or boluses in intensive care unit (ICU) patients, there are sources of fluids that may remain unrecognized but contribute significantly to the overall fluid balance. We hypothesized that fluids not ordered as boluses or maintenance infusions—“hidden obligatory fluids”—may contribute more than a liter to the fluid intake of a patient during any random 24 hours of critical care admission.Patients admitted to the Harlem Hospital ICU for at least 24 hours were included in this study (N = 98). Medical records and nursing charts were reviewed to determine the sources and volumes of various fluids for the study patients.The mean hidden obligatory volume for an ICU patient was calculated to be 978 mL (standard deviation [SD]: 904, median: 645) and 1571 mL (SD: 1023, median: 1505), with enteral feeds compared to the discretionary volume of 2821 mL (SD: 2367, median: 2595); this obligatory fluid volume was affected by a patient’s need for pressor support and renal replacement therapy.Hidden obligatory fluids constitute a major source of the fluid intake among patients in a critical care unit. Up to 1.5 L should be taken into account during daily decision making to effectively regulate their volumes.