Cell viability in peripheral blood progenitor cell (PBPC) grafts and its influence on the clinical course following transplantation was evaluated in 81 consecutive transplantations (72 autologous, 9 allogeneic) performed in patients with hematological diseases. Viability of cells in PBPC grafts immediately upon collection was 98.6 +/- 3.5%, after addition of dimethyl sulfoxide (DMSO) 73.3 +/- 21.8%, and post-thaw 65.2 +/- 16.1%. It did not differ significantly between patients with different diagnoses, gender, age, type of priming used, dose of G-CSF administered or number of CD34+ cells collected. However grafts stored for more than 60 days showed lower post-thaw viability compared to the ones thawed in the 60 days following cryopreservation (56.61 +/- 15.2% vs. 67.6 +/- 15.5%, p = 0.04). Post-thaw graft viability did not influence engraftment time, but there was a predisposition towards infectious complications in the post-transplant period in patients receiving grafts with lower percentage of viable cells. They developed febrile neutropenia more often (72.2% vs. 50% of patients, p = 0.05) and had more febrile days (2.4 +/- 2.6 vs. 1.5 +/- 2.3, p = 0.05) following transplantation. We have demonstrated that PBPC grafts are capable of long term engraftment regardless of the graft storage time or percentage of viable cells post-thaw, which confirms the robustness of CD34+ cells during the freeze/thaw procedures carried out in daily clinical practice. Granulocyte concentration in PBPC grafts could have an influence on infectious complications following transplantation and needs to be further investigated on a larger number of patients.