A patient with CD3+ large granular lymphocytic (LGL) leukemia developed transformation (TF). The phenotype of the leukemic cells was CD3+, CD4+ and CD8-. The leukemic cell count increased rapidly; the cells became large and the nuclear outline, which had been reniform, became lobulated. Anti-HTLV-1 and anti-HIV antibodies were negative in the serum of the patient and no HTLV-1 specific sequences were detected in the cDNA of the leukemic cells by polymerase chain reaction (PCR). Comparison of the karyotype abnormality of the leukemic cells before and after TF revealed an abnormality of the 21 trisomy in 90% of mitotic cells of the patient. Analysis of the cell cycle revealed that 13.7% of the leukemic cells were in DNA synthesis phase which was not previously found. The titer of anti-human herpesvirus-6 (HHV-6) immunoglobulin G which had been high at chronic phase (1:1640 compared to normal titer of less than 1:160), became 1:20,000 at TF. The titer of anti-HHV-6 immunoglobulin M also increased from less than 1:4 at the chronic phase to 1:120 at TF (normal value less than 1:4). A HHV-6-specific DNA sequence was detected by PCR in the peripheral mononuclear cells collected at TF but not at the chronic phase. These data suggests that TF occurs not only in CD3-negative but also in CD3-positive LGL leukemia. HHV-6 reactivation is therefore a possible cause in immunocompromised hosts whose general conditions are deteriorated.