Recent data suggest that the practice of using frozen allogeneic grafts is becoming increasingly common among transplant centres. Therefore, we retrospectively analysed 31 frozen allogeneic PBSC and 8 BM grafts by flow cytometry with regard to their CD34 þ content, membrane integrity (7-AAD) and stem cell-specific enzyme activity (aldehyde dehydrogenase, ALDH) in relation to individual transplantation results. Membrane integrity of CD34 þ cells was significantly impaired in cryopreserved PBSC but not in BM compared to unfrozen allografts. In 9 out of 31 frozen PBSC (but none of the BM) grafts numbers of SSC lo ALDH br cells per kg body weight (BW) were significantly reduced while in the same grafts the numbers of CD34 þ cells per kg BW were close to normal. Overall, 9 out of 33 patients (27%) who received unrelated PBSC allografts cryopreserved after transportation did not achieve engraftment. For comparison, primary graft failure was observed in our centre in only 7 out of 493 recipients (1.4%) of fresh allogeneic PBSC grafts. Moreover, we did not see any graft failure in patients receiving frozen/thawed BM or autologous PBSC transplants. We, therefore, conclude that PBSC grafts become much more sensitive to cryopreservation after transport and/or storage. Importantly, the engraftment potential of frozen HSC grafts may reliably be predicted by measuring ALDH activity.