Anti-thymocyte globulin (ATG) plus cyclosporine is effective in restoring hematopoiesis in severe aplastic anemia (SAA). We hypothesized that the humanized anti-CD52 monoclonal antibody, alemtuzumab, might be active in SAA due to its lymphocytotoxic properties. To test this hypothesis, we investigated alemtuzumab monotherapy from 2003-2010 in treatment-naïve, relapsed, and refractory SAA in three separate research protocols at the National Institutes of Health. Primary outcome was hematologic response at 6 months. For refractory disease, patients were randomized between rabbit ATG plus cyclosporine (n=27) vs. alemtuzumab (n=27); the response rate for alemtuzumab was 37% (95% confidence interval [CI], 18%-57%) and for rabbit ATG 33% (95% CI, 14%-52%; p=0.78). Three-year survival was 83% (95% CI, 68%-99%) for alemtuzumab and 60% (95% CI, 43%-85%) for rabbit ATG (p=0.16). For relapsed disease (n=25), alemtuzumab was administered in a single arm study; the response rate was 56% (95% CI, 35%-77%) and 3-year survival was 86% (95% CI, 72%-100%). In treatment-naïve patients (n=16), alemtuzumab was compared to horse and rabbit ATG in a 3-arm randomized study; the response rate was 19% (95% CI 0%-40%), and the alemtuzumab arm was discontinued early. Alemtuzumab is active in SAA but best results are obtained in the relapsed and refractory settings. The trials were registered at www.clinicaltrials.gov as NCT00195624, NCT00260689, and NCT00065260.