Aim. To study changes in the plasma concentration of Β-endorphin (Β-E) in patients with hemophilia A and B (in the presence of bleeding and in the absence of hemorrhagic syndrome) and in whole blood and plasma donors before and after donation and to investigate the factors associated with (Β-E) concentration changes. Subjects and methods. The prospective study of Β-E concentration changes (and related factors) enrolled 38 persons: 12 patients with hemophilia after acute blood loss, 11 patients with hemophilia without hemorrhagic syndrome, and 15 whole blood and plasma donors. Β-E concentrations were measured by enzyme immunoassay. Results. In blood loss, the patients with hemophilia were found to have elevated serum Β-E concentration: 9.6 pg/ml (95% confidence interval (CI), 6.1 to 13.0 pg/ml) versus 5.2 pg/ml (95% CI, 1.4 to 8.9 pg/ml). After donation, the concentration of Β-E in the group of donors was higher than before donation: 7.3 pg/ml (95% CI, 4.9 to 9.7 pg/ml) versus 4.7 pg/ml (95% CI, 3.2 to 6.3 pg/ml). In the group of patients with hemophilia, the elevation of Β-E concentrations is steady-state (lasted at least 10 days); at this time, the Β-E value variability (estimated by mean square deviation) increased as compared with that in remission: 7.7 pg/ml (95% CI, 5.5 to 13.1 pg/ml) versus 2.4 pg/ml (95% CI, 1.7 to 4.4 pg/ml). The above differences are statistically significant (p=0.05). Conclusion. In blood loss, there is an increase in plasma Β-E concentrations in the patients with hemophilia and donors. The increase in Β-E concentrations and the variability of its values were greater in the patients with hemophilia and blood loss than in the donors. The Β-E concentration elevation accompanying hemorrhage is characterized by steadiness in the patients with hemophilia.