In the Republic of Croatia it is required by law to carry out serological tests proving the presence of HBV surface antigen (HBsAg), hepatitis C virus antibody (anti - HCV), antigen / antibody Human Immunodeficiency Virus (HIV Ag / At) and antibodies to the syphilis (anti-treponema pallidum) agent at each blood donation, and from March 1st 2013, Multiplex individual donor nucleic acid testing (ID - NAT) HBV - DNA, HCV - RNA, HIV - 1 - RNA as well. Study Design The study was designed as an observational study – intersected with historical data in the period from March 1st 2013 to March 1st 2017. Participants and methods From March 1st 2013 to March 1st 2017, 743 490 blood samples of multiple and new volontary blood donors (VBD) in eight authorized health institutions for blood collection were tested by serological and molecular methods. The results of the testing are linked to the blood donor and blood supply through the e-Delphyn National Information System of the Blood Transfusion Service of the Republic of Croatia. Results In this period, 56 cases of occult HBV infection (OBI) VBDs and 1 potential transmission of HBV DNA to the recipient of blood products were detected in the Croatian blood transfusion service. There was 25 look-back and no trace - back procedures. OBI VBDs were mostly men over 50 years of age, and there was no significant difference between multiple and new VBDs in the incidence of OBI. Anti - HBc marker was positive in 96% OBI VBDs. The level of detection of HBV DNA in 92% of cases was below the limits of detection or very low (< 20 IU / mL). The consultation was attended by 75% of OBI VBDs where their health status was assessed as satisfactory (healthy) and all OBI VBDs were reported to the appropriate epidemiological services. Conclusion Blood transfusion is mostly safe and successful procedure but side effects still happen.