Eliason MJ - Among patients undergoing endoscopic sinus surgery (ESS), the researcher determined the efficiency of intraoperative blood loss estimates by attending otolaryngology surgeons. Data were obtained on six surgeons performing ESS at a military academic medical center for 21 surgical cases after obtaining institutional review board approval. Specifically, both hourly and end-of-case total “estimated” (EBL) and “calculated actual” (ABL) blood loss values have been recorded and contrasted statistically. Findings revealed that the difference between mean EBL and ABL was 62.5 ml and was statistically important. The surgeons examined had EBL which was statistically significantly lower than ABL at both the hourly intervals during the operation and at the end of the case. As a consequence, in clinical care and in medical research/advancement activities there is potential for adverse effects.