Summary:Peripheral blood stem cell transplantation after reducedintensity conditioning (RIC-PBSCT) regimen is an alternative to conventional regimens with less immediate toxicity. Since immune recovery is of crucial importance for the control of infections, we retrospectively studied the recovery of T-, B-and NK cell subsets in 20 consecutive patients undergoing RIC-PBSCT. We also studied the thymic output using T-cell receptor excision circle assay. Engraftment was rapid and few infectious complications were seen: three early (before 2.5 months) cases of asymptomatic cytomegalovirus reactivation, two late Gram-negative bacterial infections and no fungal infection. While CD4 þ T-cell reconstitution was slow, CD8 þ T-cell counts were close to normal values at 4 months. Median CD19 þ B-cell counts reached normal values at 11 months. Rapid CD56 þ NK cell reconstitution was noticed as early as 1.5 months. Low T-cell receptor excision circle numbers and preponderance of memorytype subsets among T cells further suggested that CD8 þ T-cell reconstitution resulted predominantly from peripheral expansion and that thymic-dependent reconstitution was severely impaired. In conclusion, large peripheral Tcell expansion may compensate for late thymic-dependent lymphopoiesis, and may, with other factors such as NK and B-cell reconstitution and careful antiinfectious prophylaxis, help limit the incidence of severe infections after RIC-PBSCT. Bone Marrow Transplantation (2005) 35, 859-868.