In a phase I–II study for patients aged 55–65 years, we employed radioimmunotherapy using an anti-CD-66 antibody as part of a dose-reduced conditioning regimen, which was followed by a T-cell-depleted graft. 20 patients with a median age of 63 years suffering from acute leukaemia (n = 17) or myelodysplastic syndrome (n = 3) received the antibody labelled either with Rhenium (n = 8) or with Yttrium (n = 12) during conditioning. Radioimmunotherapy provided a mean dose of 21·9 (±8·4) Gy to the bone marrow with a significantly higher dose when Yttrium was used. Additional conditioning was fludarabine-based plus anti-thymocyte globulin in matched related donor transplants (n = 11), or plus melphalan in matched unrelated donor transplants (n = 9). Regimen-related toxicity was low, with two patients developing three episodes of grade III organ toxicity. All patients engrafted, grade II–IV acute graft-versus-host disease (GvHD) was observed in one patient (5%) and chronic GvHD in three patients (15%). The cumulative incidence of non-relapse mortality was 25%, the cumulative incidence of relapse 55%. The probability of survival was estimated to be 70% at 1 year and 52% at 2 years post-transplant, although no plateau was reached afterwards. In conclusion, radioimmunotherapy using the anti-CD66 antibody was feasible and safe in our elderly patient group and provided a high marrow dose.