Not all clinical laboratories have an osmometer, and calculations for osmolality are a frequently used method for determining osmolality. The purpose of this study was to evaluate the performance of four formulas for the estimation of osmolality, with cryoscopic measurement as the reference standard in intracranial hemorrhage (ICH) and head injury (HI) patients who were not treated with mannitol. Forty III and 31 ICH patients treated in the Neurosurgery Department were included in the study. Every 6 h over a period of 24 h, serum samples were collected from patients and osmolality was measured. In conclusion, our study shows that only formulas F1 [Osmolality = 1.86(Na) + 1.86(K) + Glucose + Urea] and F4 [Owsmolality = 1.86(Na) + Glucose + Urea + Ethanol + 9] can be used to evaluate osmolality In ICH patients who were not treated with mannitol. In III patients, none of the formulas should be used to calculate osmolality. (c) 2009 Elsevier Inc.