Delayed immune reconstitution following allogeneic stem cell transplantation remains a major clinical problem, resulting in significant transplant-related mortality from infectious complications. The recovery of immunity after bone marrow transplantation (BMT) is a complex process dependent on a large number of pre- and post-transplant factors. It has been suggested that the use of peripheral blood instead of bone marrow as stem cell source may accelerate immune reconstitution after allogeneic transplantation. Some authors have recently reported a more rapid recovery of the number and function of T and B cells after allogeneic peripheral blood progenitor cell transplant (allo-PBPCT) in comparison with conventional BMT, results which would reflect the high number of lymphocytes infused to the patients. Such a rapid immune recovery could indeed contribute to the apparent therapeutic advantage of PBPCT when compared with BMT. However, there is limited knowledge on this issue and randomized trials are required to prove whether allo-PBPCT is indeed superior to BMT in terms of immune reconstitution post-transplant. A review of some quantitative and functional aspects of immune recovery after allo-PBPCT is presented in this article.