This study detected clonal T cells in patients with acquired pure red-cell aplasia (PRCA) by Southern blotting and polymerase chain reaction (PCR). Twenty-nine adult patients with acquired PRCA were enrolled in this study. Seventeen patients had primary acquired PRCA, while 12 patients had the secondary form. Twenty-two of 29 (76%) patients demonstrated TCR rearrangement by at least one method. We divided the patients into three groups depending on T-cell clonality. The CD4/8 ratio of patients who were positive on Southern blotting was significantly lower than that of other groups. Except for the CD4/8 ratio, other laboratory findings did not significantly differ among the three groups. The CD4/8 ratio should be a useful surrogate marker to detect T-cell clonality. Am. J. Hematol. 79:332–333, 2005. © 2005 Wiley-Liss, Inc.