The possibility of infection transmission by infusion of cryopreserved peripheral blood stem cells concentrates or bone marrow is well known. For this reason the European Blood and Marrow Transplantation Group (EBMT) and International Society for Haemotherapy and Graft Engineering (ISHAGE) standards include a panel of serological tests to be performed in donors and patients with the aim to lower the likelihood of infection transmission. The study was performed on a group of 71 patients and 22 donors. No laboratory signs of active infection were found in 15 donors (13 related, 2 unrelated), i.e., in 68.2% and in 55 patients (77.5%). The active infection from herpes viruses was the most common (in patients 13, in donors 7). Hepatitis B was found in only one case. The cytomegalovirus (CMV) immunoglobulin G (IgG) test was the most common marker of previous infection, and it was found in 14 donors and 55 patients. We can conclude that the rate of clinically unsuspected infections in donors and patients, including cases requiring immediate treatment among the patient groups, is relatively high and fully justifies the practice of prophylactic serological testing using the whole palette of tests according to EBMT and ISHGE in both autologous and allogenous transplantations of hematopoietic stem cells.