Sublethal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) produces thymic atrophy and thymic lymphodepletion in all species investigated. Lymphodepletion is first seen in the cortex of the thymus, but also occurs in the medulla at later time points. This article surveys in vivo and in vitro studies on TCDD-induced thymus toxicity. It is concluded that the first intrathymic target cell for the action of TCDD is the cortical epithelial cell.