ATLL is etiologically associated with HTLV-I retrovirus. A population of 10 to 20 million worldwide is estimated to be infected by the virus, but only 1-4% develop ATLL during a 70-year lifespan. The latency period is more than 30 years. The aim of this study was to report two cases of ATLL in Greek patients with the concomitant study of their family members. A 55-year-old woman and a 59-year-old man presented with leucocytosis and lymphocytosis. Both were asymptomatic and physical examination was unremarkable except for minimal lymphadenopathy in the second patient. In both patients blood smears showed small-to-medium-sized, multilobulated lymphocytes, with different degrees of nuclear irregularity. Immunophenotypic study was as follows: CD2 + (97%), CD3 + (95%), CD5 + (95%), CD3/CD4 + (93%), CD3/CD25 + (84%), CD7 -/CD4 + (89%) CD2 + /HLA-DR + (53%), TCRabeta + (96%) and CD7-(7%). Bone marrow biopsy revealed a normal cellularity with dyserythropoiesis and scattered small lymphocytes (CD4 + on immunostaining) Serum HTLV I and II antibodies were positive. T-cell receptor gamma-chain rearrangement was positive in blood lymphocytes by PCR. Cytogenetic analysis showed complex karyotypic abnormalities. DNA analysis by PCR demonstrated the integration of the HTLV-I DNA in the DNA of the neoplastic T cells. Both patients rapidly developed acute type ATLL. In the first patient multiple subcutaneous nodules on the palmar surface of both hands were also observed. She received deoxycoformycin, which was stopped because of autoimmune hemolytic anemia. Corticosteroid treatment was initiated, with gradual improvement. She suffered from recurrent opportunistic infections. She is currently under interferon and zidovudine therapy with stable blood parameters. Chemotherapy was administered to the other patient with > 50% initial response. Both patients' families were tested for serum anti HTLV-I antibodies and their mates were found to be positive; they also had detectable viral DNA by PCR analysis while asymptomatic, with no abnormal clinical findings and normal white blood cell count and morphology. In conclusion, the two aforementioned patients are the first fully documented ATLL patients described in Greece. Investigation for HTLV-I antibodies should be mandatory in all patients with T-cell lymphoproliferative disorders.