We investigated the occurrence of late events (beyond 2 years) in patients with acute leukaemia who received an allogeneic (BMT) (n = 1059), or an autologous bone marrow transplantation (ABMT) (n = 656) in Europe during the period from January 1979 to December 1990. Patients with no recurrence of leukaemia at 2 years had overall 82% chance of being alive in complete remission at 9 years following transplantation regardless of the nature of the leukaemia, the status at transplant, and the type of transplant. The incidence of late relapses continuously decreased with time. The latest relapses in acute myelogenous leukaemia (AML) were observed following BMT at 6.6 years in a patient transplanted in first remission (CR1) and at 3.7 years in a patient transplanted in second remission (CR2), and following ABMT at 6 years and 5.1 years respectively. The latest relapses in acute lymphoblastic leukaemia (ALL) were observed following BMT at 4 years in a patient transplanted in first remission (CR1) and at 6.8 years in a patient transplanted in second remission (CR2), and following ABMT at 5.3 years and 4.5 years respectively. Several factors predictive for late relapse or death were identified. Patients allografted experienced a lower frequency of late relapse than patients autografted. Of the numerous other prognostic factors studied, female sex in AML, the use of total body irradiation (TBI) in ALL and status in CR1, rather than CR2-3, for both ALL and AML allografted were correlated with a lower relapse incidence. The use of TBI in ALL was also associated with a better LFS and survival. The absence of acute graft-versus-host disease (GVHD) in allografted AML correlated with better LFS and better survival, but had no influence on the relapse incidence. This study indicates that patients alive and well at 2 years post transplant have a very high probability of being cured, but the possibility of late relapse still remains.