Purpose: The purpose of the study was to evaluate changes of hematologic parameters in bimaxillary surgery. Methods: Fifty-three patients were prospectively evaluated and divided into groups based on the surgical procedure and sex (predictor variables). Hemoglobin, red blood cells, hematocrit, and platelet were the primary outcome variables, operation time the secondary outcome, and the patients’ age and weight the other variables. Trial registration: NCT02364765 (U.S. National Institutes of Health, clinicaltrials.gov). Results: There was statistically significant difference between all hematologic parameters before and after surgery, for both men and women, and for all surgical groups. There was a positive correlation between operative time and the decrease (in %) of the hematologic parameters. Linear regression analysis suggested that the Hb values decrease 0.083 % for every minute increase in the operation time, and 0.066, 0.066, and 0.010 % for RBC, Hct, and platelet count, respectively. There was a negative correlation between weight and all hematologic parameters. Correlations between age and hematologic parameters were not statistically significant. Almost all correlations between age, weight, sex, and the surgery group and the hematologic parameters were considered as very weak. Only one patient was transfused. Conclusions: It is suggested that operation time and patient’s weight play a bigger role than patient’s age and sex in the decrease of hematologic parameters after bimaxillary surgery.