The collection of plasma samples from all patients for HHV-5 PCR investigations, according EBMT guidelines, began at a median of 3 days after transplantation (range: 1-7 days) and lasted until a median of 105 days (range: 30-180 days). Routine collection of plasma samples was performed once a week until the 100th day after allogeneic HSCT, thereafter once every two weeks, up to 180th day after HSCT. The median number of blood samples per patient was 12 (range: 3-21). A total of 294 samples from the 26 patients were collected. For the purpose of recent study, viral DNA was extracted from 200 µl of each plasma sample using a QIAmp ® DNA Blood Mini Kit (Qiagen) in accordance with the manufacturer's instructions and retrospectively used for detection of HHV-6 DNA.