Few reports on the successful treatment of T-cell large granular lymphocyte (LGL) leukemia with the humanized anti-CD52 monoclonal antibody alemtuzumab are emerging in the literature. The expression of CD52 by LGLs has not been previously investigated. Using semi-quantitative 2- and 3-color flow cytometry, we documented the expression of CD52 in 100% of abnormal cells in T-cell LGL leukemia (n = 11) and natural killer (NK) cell LGL leukemia (n = 2), and showed no significant difference in CD52 expression between T-cell prolymphocytic leukemia (PLL) and T-cell LGL leukemia. Higher CD52 expression has been noted in responders to alemtuzumab in T-cell PLL and in chronic lymphocytic leukemia (CLL), a B-cell disorder. The strong and consistent expression of CD52 shown here highlights the potential role of alemtuzumab in the treatment of refractory T-cell LGL leukemia and possibly aggressive NK cell leukemia.