Abstract: Relevance . In accordance with the guidelines on the clinical investigation of clotting factor VIII products of the European Medicines Agency and guidelines on pharmacovigilance of the Eurasian Economic Union, after registration of a new drug, it is recommended to study its efficacy and safety on a large population of patients in a standard medical practice to clarify and identify new data. Materials and methods . In a prospective, multicenter, open-label, uncontrolled observational study, the efficacy and safety of the domestic recombinant B-domain deleted blood clotting factor FVIII (FVIII) (moroctocog alfa, Octofactor®, JSC “GENERIUM”) in patients with moderate and severe hemophilia A in the context of standard medical practice (study protocol number CI-51/15). Patients received the drug in terms of standard medical practice for the purpose of prophylactic treatment or on demand treatment. For prophylactic treatment Octofactor was administered to patients according to the instructions for medical use in a single dose of 20–40 IU/kg every 2–3 days. In the case of bleeding a single dose of Octofactor was calculated taking into account the severity and localization of bleeding in accordance with the instructions for medical use. The results of the treatment were analyzed for a period of 52 ± 2 weeks. The main parameter for evaluating the efficacy was the frequency of spontaneous bleeding that occurred within 48–72 hours after the administration of the Octofactor. Additional parameters for evaluating the efficacy included: the severity of spontaneous bleeding arising during the prophylactic treatment; the number of injections and the total dose of the Octofactor to stop 1 episode of bleeding; the amount of Octofactor used during the entire observation period (52 ± 2 weeks) and for 1 month both for prophylaxis and for stopping the bleeding that occurred; an indicator of the efficacy of therapy on the scale for determining the response to treatment of acute hemarthrosis (World Federation of Hemophilia, WFH). Results. According to the results of the screening survey 237 male patients aged from 19 to 78 years old (mean age 35.2 ± 11.1 years) with moderate and severe hemophilia A (FAS-population) were included in the study. The efficacy of therapy was evaluated in 202 patients who underwent all the planned procedures during the observation period (PP-population). 193 (95.5 %) patients received prophylactic treatment, 9 (4.5 %) patients received on-demand treatment. Evaluation of the efficacy of treatment was carried out on the basis of basic and additional parameters. The main parameter for evaluating the efficacy – the frequency of spontaneous bleeding that occurred within 48–72 hours after the administration of the Octofactor – was 52 ± 2 weeks within 1.4 ± 2.9 cases. At the same time, the proportion of spontaneous bleeding that occurred within 48–72 hours after administration of the Octofactor preparation was 45.2 % of the total number of spontaneous bleeding and 15.6 % of the total number of all bleeding in patients who received prophylactic treatment. Among 608 spontaneous bleeding that occurred in patients receiving prophylactic treatment, 287 (47.2 %) of the bleeding were mild, 289 (47.5 %) were moderate and 32 (5.3 %) were heavy. Of the 275 spontaneous bleeding that occurred within 48–72 hours after administration of the study drug for prophylactic purposes, 117 (42.5 %) episodes were mild, 146 (53.1 %) were moderate, and 12 (4.4 %) were severe. With prophylactic administration the average single dose of the Octofactor was 2036.3 ± 884.7 IU, or 27.3 ± 11.2 IU/kg, in the treatment of bleeding occured during prophylactic treatment – 2227.7 ± 1087 IU, in the treatment of bleeding in patients receiving the drug only on demand – 2280.7 ± 1037.2 IU. The average monthly intake of the drug by one patient in prophylactic treatment was 19.75 ± 9.75 thousand IU, while the average monthly consumption of the drug for preventing bleeding from one patient was 17.16 ± 9.13 thousand IU for stopping bleeding against the background prevention – 3.87 ± 3.97 thousand IU. One patient who received on-demand treatment had an average monthly average of 13.47 ± 13.46 thousand IU of the Octofactor preparation. For stopping 1 bleeding, on average, 1.7 ± 1.7 injections of the Octofactor preparation were required, in the prophylactic treatment group – 1.8 ± 1.8, and in the on-demand treatment group – 1.5 ± 1.1. In the overwhelming majority of cases, patients of both groups showed excellent and good response to all treatment of acute hemarthrosis on the scale of the WFH on all visits, the reaction was moderate in a few episodes, and only in 1 case of acute hemarthrosis there was no response to the drug administration. The safety of therapy was evaluated in 228 patients who received at least 1 Octofactor administration during the study (mITT-population). There were 66 adverse events in 40 patients, 10 of them were associated with the use of the drug, the most significant of which were the formation of inhibiting antibodies to FVIII in low titer (1.5 U) in 1 patient and the development of allergic reactions in 2 patients. Conclusions. Under the conditions of standard medical practice the efficacy and safety of Octofactor was confirmed for both prophylactic treatment and on-demand bleeding treatment in adult patients with severe and moderate hemophilia A.