Patients with aplastic anemia or hypoplastic myelodysplastic syndrome (MDS) may respond to immunosuppressive therapy, including the anti-CD52 antibody alemtuzumab. We analyzed treatment responses to alemtuzumab in 5 patients with MDS or aplastic anemia (AA) evolving to MDS. Two patients with hypoplastic MDS (hMDS) showed a partial response (PR) to alemtuzumab. In both responding patients, a concomitant paroxysmal nocturnal hemoglobinuria (PNH) clone was detected before therapy. One responder relapsed after 15 months and underwent successful allogeneic stem cell transplantation. Both patients are still alive and in remission after 40 and 20 months, respectively. The other patients showed no response to alemtuzumab. One patient died from pneumonia 4 months after treatment. In summary, our data confirm that alemtuzumab is an effective treatment option for a subset of patients with MDS, even in the presence of a PNH clone.