There is a growing interest in the use of peripheral blood stem cells (PBSC) for allogeneic transplantation. This is due in part to the idea that, as with autologous transplantation, increasing the number of allogeneic hemopoietic progenitors infused may lead to reduced complications. However, introducing the PBSC technique into allogeneic transplants implies theoretical as well as ethical problems involving both patient and donor. We are still uncertain whether the PBSC technique will result in an increase of GVHD or (better) of GVL. G-CSF, necessary for effective PBSC mobilization, is safe but its use in normal subjects should be regarded with caution. For this reason, a Study Committee promoted by the GITMO (Gruppo Italiano Trapianto di Midollo Osseo) evaluated the key aspects of allogeneic PBSC collection and transplantation. The present paper summarizes the scientific data and suggests some guidelines for the introduction of allogeneic PBSC transplantation into clinical practice. The procedure should be considered experimental and the Committee strongly recommends the use of allogeneic PBSC in experienced centers, initially in patients with advanced disease. The donor should be given a complete explanation of the advantages and risks of G-CSF therapy, leukapheresis and general anesthesia. A careful monitoring of both patient and donor should also be included to watch for short-term and long-term side effects.