Comparative study of cutaneous T-cell lymphoma and adult T-cell leukemia/lymphoma. Clinical, histopathologic, and immunohistochemical analyses. An important disease entity distinct from cutaneous T-cell lymphoma (CTCL) in Japan is adult T-cell leukemia/lymphoma (ATL), which usually shows the same phenotype as CTCL, i.e., a helper/inducer T-cell phenotype (CD4+CD8-), and usually involves the skin. Clinically, both CTCL and ATL are heterogeneous in nature. In this study, we demonstrated differences between CTCL and ATL in terms of clinical and immunopathologic cell surface features. In patients with ATL, the predominant clinical findings were peripheral lymph node involvement, skin lesions, hepatosplenomegaly, leukemic manifestations, and an aggressive course. In patients with CTCL, by contrast, only skin lesions predominated at the onset of the disease and a relatively good prognosis was demonstrated. Phenotypic heterogeneity of ATL in the skin, i.e., CD4-CD8-, CD4+CD8-, and CD4-CD8+, was demonstrated. Expression of Leu8, CD7 (Leu9), and CD45RA (2H4) was high in both the skin-infiltrating ATL cells and peripheral blood and lymph node ATL cells compared with that in the skin-infiltrating CTCL cells. Expression of CD25 (IL-2R), CD71 (OKT9), HLA-DR, and HLA-DQ was higher in the skin-infiltrating ATL cells than in CTCL cells. Expression of CD29 (4B4) was high, and that of CD45RA (2H4) was low in both the skin-infiltrating ATL and CTCL cells compared with the peripheral blood and lymph node ATL cells. Expression of CD45RO (UCHL-1) was not significantly high in the skin-infiltrating CTCL cells compared with that in ATL cells. The most significant phenotypic difference between ATL cells and CTCL cells was the expression of Leu8 (lymph node homing receptor), CD7 and CD25 antigens on the cell surface, and the main phenotypic difference between skin-infiltrating ATL and CTCL cells and peripheral blood and lymph node ATL cells was the expression of CD29 and CD45RA. These findings confirm that the difference in antigen expression on the cell surface might reflect the clinical features of ATL and CTCL, and suggest that the predominant phenotype of peripheral blood and lymph node ATL cells is that of naive, relatively immature or activated T-cells, and that CTCL cells are previously activated (memory) T-cells. In other words, CTCL cells do not share the same origin as ATL cells. These observations support the concept that ATL is a disease distinct from CTCL. 