Adult T-cell leukemia/lymphoma(ATL) was first discovered and reported in Japan, where it has a high incidence in the southwestern region of the country. The retrovirus human T-cell leukemia virus type I(HTLV-I) is considered to be related to its etiology. ATL shows divers clinical features. It can be divided into four types of smoldering, chronic, acute, and lymphoma. ATL cells originate from the CD4-positive subset of peripheral T cells showing a characteristic notch in the nucleus and a tendency for lobulation. A definit diagnosis of ATL is made by documenting the presence of HTLV-I proviral DNA in the DNA of leukemic or lymphoma cells. Crinico-Pathological aspects of ATL are more complexed than other types of lymphoma because of the verity of the disease type, state of immunodeficiency, hypercalcemia, cytokine activation, and so on.