We currently showed that UV-induced tolerance is mediated by the induction of a new type of T suppressor cells, called CD4⁺CD25⁺ T regulatory cells (Treg), which are recently highlighted to counteract the onset of various autoimmunity, including experimental autoimmune encephalomyelitis or type 1 diabetes. Treg, in general, ultimately require to be maturated in thymus, leading to a concept of central tolerance. Although the involvement of Treg in UV-induced tolerance is no doubt, it is still uncertain how and where UV-induced Treg are generated. To address this issue, we generated athymic C3H mice, which were thymectomized at an age of day 3 and grown to 8–10 wk. Using athymic mice we tested the involvement of thymus in contact hypersensitivity (CHS) and its modulation by UV. Athymic mice showed vigorous CHS response, indicating that the induction of T effector cells (Tef) was not impaired. However, UV-induced tolerance was not observed in athymic mice, suggesting requirement of thymus in this process. Even when T cells obtained from naive animals were transferred intravenously to athymic mice, UV-radiation failed to induce tolerance, further confirming the significant role of thymus in this process. Upon transplantation of age-matched thymus into kidney capsules of adult athymic mice, they regained the ability to undergo UV-induced tolerance. Collectively, the present study demonstrates the involvement of thymus in UV-induced tolerance and thereby suggests that UV-induced tolerance is acquired central (thymic) tolerance.