We have studied the reconstitution of the T-cell compartment after bone marrow transplantation (BMT) in five patients who received a graft-versus-host disease (GVHD) prophylaxis consisting of methotrexate, cyclosporin, and 10 daily injections (day −4 to day +5) of Campath-1G. This treatment eliminated virtually all T cells (7 ± 8 T cells/μL at day 14) which facilitated the analysis of the thymus-dependent and independent pathways of T-cell regeneration. During the first 6 months, the peripheral T-cell pool was repopulated exclusively through expansion of residual T cells with all CD4⁺ T cells expressing the CD45RO-memory marker. In two patients, the expansion was extensive and within 2 months, the total number of T cells (CD8>>CD4) exceeded 1,000/μL. In the other three patients, T cells remained low (87 ± 64 T cells/μL at 6 months) and remained below normal values during the 2 years of the study. In all patients, the first CD4⁺CD45RA⁺RO⁻ T cells appeared after 6 months and accumulated thereafter. In the youngest patient (age 13), the increase was relatively fast and naive CD4⁺ T cells reached normal levels (600 T cells/μL) 1 year later. In the four adult patients (age 25 ± 5), the levels reached at that time-point were significantly lower (71 ± 50 T cells/μL). In all patients, the T-cell repertoire that had been very limited, diversified with the advent of the CD4⁺CD45RA⁺RO⁻ T cells. Cell sorting experiments showed that this could be attributed to the complexity of the T-cell repertoire of the CD4⁺CD45RA⁺RO⁻ T cells that was comparable to that of a normal individual and that, therefore, it is likely that these cells are thymic emigrants. We conclude that after BMT, the thymus is essential for the restoration of the T-cell repertoire. Because the thymic activity is restored with a lag time of approximately 6 months, this might explain why, in particular in recipients of a T-cell–depleted graft, immune recovery is delayed.