Peripheral blood lymphocytes of 46 recipients of lymphocyte-depleted bone marrow allografts were phenotypically analysed over a period of 1 year. We investigated the repopulation of lymphocyte subpopulations and their relation with clinical parameters such as graft-versus-host disease (GVHD), graft-versus-leukaemia and cytomegalovirus (CMV) infection. The number of repopulated T cells varied strongly between the blood samples of the recipients. In 45% of the recipients the number of T cells recovered to or above normal levels within 3 months after bone marrow transplantation (BMT), whereas the other recipients remained below normal up to 1 year after BMT. In recipients with a high repopulation, the CD8+ T-cell subset contributed more to this high repopulation than the CD4+ T-cell subset. We showed that the majority of T cells of these recipients expressed the alpha beta T-cell receptor, CD8, CD57 and CD11b. HLA-DR was also highly expressed reflecting the activation stage of T cells in these recipients. BMT recipients with a high repopulation of CD8+ T cells showed a lower incidence of leukaemic relapse than recipients with a low repopulation. The 3-year probability of relapse was 19% versus 64% (P = 0.03), respectively. The relative high number of CD8+ T cells at 3 months after BMT was not associated with the incidence of GVHD. In contrast, occurrence of CMV infection after BMT was significantly higher in these recipients. Our results indicate that CD8+ T cells, predominantly CD57+, of BMT recipients with an expansion of these cells represent an in vivo activated cell population. This CD8+ T-cell population may consist partially of cytotoxic cells with anti-leukaemic activity as suggested by a low relapse rate. The signal for the strong expansion of these CD8+CD57+ T cells after BMT is still unclear, but association with CMV infection suggests that viral antigens are involved.