Red blood cell distribution (RDW) is a hematologic index automatically calculated by blood cell counters. Research about RDW in traumatic brain injury showed positive correlation between high RDW values and mortality, which inspired us to investigate whether RDW could be used as a supportive diagnostic biomarker for diagnosis of brain death. Our hypothesis is that RDW may be useful as a biomarker that supports the diagnosis of brain death.After approval of the ethics committee, 209 patients who had been diagnosed with brain death between January 2012 and July 2018 were retrospectively reviewed. The RDW values of patients on the days of admission, brain death, and cardiac arrest were recorded. Data were collected from hospital database and patient charts.Statistical analysis revealed that the RDW values on the days of brain death and cardiac arrest were significantly higher than on the day of admission. In addition, the RDW values for the cardiac arrest day were significantly higher than on the day of brain death (P < .001).We can say that the increase in RDW, which is reported to be an indicator of mortality for many diseases, can be a supporting biomarker for brain death diagnosis when evaluated concomitantly with clinical diagnostic criteria.