This retrospective study concerns non-bacterial infections in Asian patients receiving alemtuzumab. The clinical data of 182 patients treated with alemtuzumab alone or alemtuzumab-containing chemotherapy between the years 2003 and 2009 was collected from six Asian countries. Alemtuzumab was used in the setting of frontline (n =48) or salvage (n =90) treatment, and as a part of the conditioning regimen for allogeneic stem cell transplant (n =44). Reactivation of cytomegalovirus (66/182) and varicella zoster virus (25/182), and fungal infection (31/182) including invasive pulmonary aspergillosis, were the most common infectious complications in this retrospective analysis. Thus, we recommend routine prophylaxis with valganciclovir and itraconazole, especially when alemtuzumab is used in the conditioning regimen for allogeneic stem cell transplant. Pneumocystis jirovecii pneumonia (PJP) was found in four patients (3%, 4/122) receiving alemtuzumab as conditioning for stem cell transplant or salvage treatment. Three cases of hepatitis B virus reactivation were found in antigen-negative patients, and 16 cases of tuberculosis were observed. Infection is the major complication of alemtuzumab therapy, and these infectious complications are potentially severe and life-threatening. Based on our retrospective analysis, we have constructed a guideline for antimicrobial prophylaxis in Asian patients receiving alemtuzumab therapy.