The role of reduced-intensity conditioning allogeneic stem cell transplant (RIC allo-SCT) in aggressive B-cell non-Hodgkin lymphoma (NHL) remains a matter of debate. This single-center analysis aimed to assess the potential benefit of RIC allo-SCT in 19 consecutive patients with relapsed or transformed aggressive B-cell NHL. Aggressive transformation (primary or secondary) was documented for these patients by pathological examination. In this series, all patients but two (n = 17; 89.5%) could actually receive autologous stem cell transplant (auto-SCT) prior to RIC allo-SCT. At the time of allo-SCT, eight patients (42%) were in first complete remission (CR), six (31.5%) were beyond first CR, and five (26.5%) were in partial remission. With a median follow-up of 32 (range, 3-86) months, nine patients experienced grade 2-4 acute GVHD (47.5%) and 10 patients had extensive chronic GVHD (52.5%). Overall, the incidence of non-relapse mortality was 26% (95% CI, 8-44%). At last follow-up, 12 patients (63%) were in sustained CR. The Kaplan-Meier estimates of progression-free and overall survival rates were 68% and 68%, respectively, at 4 years. We conclude that RIC allo-SCT after auto-SCT is feasible and a potentially efficient therapy for relapsed or transformed aggressive B-cell NHL, warranting further prospective evaluation.