We describe a 57-year-old man, affected by large granular lymphocyte (LGL) leukemia and concomitant primary progressive multiple sclerosis (MS), treated with allogeneic hematopoietic stem cell transplantation (HSCT) from an HLA-identical sibling. The patient was conditioned with fludarabine, busulphan, and cyclophosphamide. Graft-versus-host disease (GVHD) prophylaxis consisted of cyclosporine and short-term methotrexate. At 3 years follow-up, the patient is in complete remission of LGL with a marked improvement in neurological conditions. This is the first case of allogeneic HSCT in a patient with LGL leukemia and concomitant primary progressive MS. Allogeneic HSCT, performed in our patient to cure the lymphoproliferative disorder, improved the clinical course of MS.