We report the clinical, hematologic, and immunologic features of 18 preleukemic adult T cell leukemia (pre-ATL) cases with abnormal T lymphocytosis induced by human adult T cell leukemia-lymphoma virus (HTLV/ATLV). The patients were from the Nagasaki district, which is one of the most endemic areas of ATL in Japan. Pre-ATL is a subclinical T cell abnormality differing from ATL. It is characterized by an insidious onset and appearance of abnormal T lymphocytes (10% to 40%) in the peripheral blood without clinical symptoms except for a few cases transiently presenting fever, skin eruptions, and slight lymphadenopathies. Most abnormal T lymphocytes were small and mature with incised or lobulated nuclei and formed E rosettes with sheep RBCs. Virologic and biomolecular analysis revealed that all cases were infected with HTLV, and proviral DNA was integrated in host lymphocytes from 12 of the 14 cases examined. Furthermore, the lymphocyte populations, including abnormal T lymphocytes, were monoclonal with respect to the site of the provirus integration. Abnormal T lymphocytosis persisted from one to more than seven years in six cases, three of which developed ATL after a one- to five-year pre-ATL stage, whereas abnormal T lymphocytes spontaneously decreased in the other seven patients. However, HTLV-infected monoclonal lymphocytes were detected in four cases examined, even after most of the abnormal T lymphocytes had disappeared. Moreover, the same clonally provirus-integrated lymphocytes persisted in two of four cases not only during the course of abnormal lymphocytosis, but also in the subsequent almost-normal blood. These results indicate that the majority of the cases were in a pre-ATL state with a potential to develop ATL.