There have been only three reports in the literature of T-cell large granular lymphocyte (T-LGL) leukemia occurring after autologous peripheral stem cell transplantation (APBSCT). We describe 3 patients in whom a transient monoclonal T-LGL developed after APBSCT for malignant lymphoma. Case 1: A 58-year-old man with peripheral T-cell lymphoma in second complete remission (CR) who underwent APBSCT. Case 2: A 51-year-old man with follicular lymphoma in second CR who underwent APBSCT. Case 3: A 65-year-old man with diffuse large B-cell lymphoma in second CR who underwent tandem APBSCT. One month after transplant, fever followed by the proliferation of CD8+/CD57+ T-LGL in peripheral blood occurred in all three cases. Because clonal rearrangements of the T-cell receptor were detected in peripheral blood samples, T-LGL leukemia was diagnosed. The first patient had episodes of Epstein-Barr virus viremia. The other patients suffered from cytomegalovirus colitis after APBSCT. These data show that T-LGL leukemia can occur after viral infection followed by APBSCT.