Adult T-cell leukemia/lymphoma is an aggressive malignant disease associated with regulatory T cells as discussed in some recent reports. We analyzed the expression of FOXP3, a key molecule of regulatory T cells, in adult T-cell leukemia/lymphoma and its association with clinicopathological features. Of 169 adult T-cell leukemia/lymphoma cases examined, 60 (36%) showed FOXP3 expression in lymphoma cells. Morphologically, 22 cases were classified as anaplastic large cell variant and 147 as pleomorphic cell variant. Only 1 (5%) of the anaplastic large cell variant cases and 59/147 (40%) of the pleomorphic cell variant cases expressed FOXP3. Epstein-Barr virus-infected cells were significantly more frequently found in FOXP3(+) cases (23/60; 38%) than in FOXP3(-) cases (12/109; 11%) (P<0.0001). Cytogenetic analysis showed that FOXP3(+) cases had simpler chromosomal abnormalities than FOXP3(-) cases. Clinically, FOXP3(+) and FOXP3(-) cases did not differ significantly in age distribution, clinical stage, lactate dehydrogenase and calcium in serum and overall survival. However, 8 of 34 FOXP3(+) cases suffered a severe infectious state, an indication of immunosuppression, while only 2 of 62 FOXP3(-) cases did so (P<0.005). FOXP3 expression in adult T-cell leukemia/lymphoma thus reflects morphological features and is clinically and pathologically associated with an immunosuppressive state.