We previously reported that lycopene, one of the carotenoids, significantly suppressed the development of spontaneous mammary tumors of mice and the contribution of lycopene to the maintenance of physical homeostasis was suggested. In this study, we investigated the effects of lycopene on the intrathymic and peripheral T cell subpopulation as immunoregulatory parameters. In tumorous control mice, inactivation of intrathymic T cell differentiation, an increase of the percentage of CD4+ CD8+ cells and a decrease of CD4+CD8+ cells were observed. On the other hand, in the lycopene-treated mice, the abnormal intrathymic T cell differentiation was recovered and brought to non-tumorous levels. The peripheral CD4/CD8 ratio was partially augmented by lycopene treatment which resulted from an increased CD4+ subpopulation. These observations suggest that one of the immunomodulating roles of lycopene is to normalize the change of intrathymic T cell differentiation caused by tumorigenesis.