Recently evidence has accumulated that atopic disease is associated with a deficiency of thymus-derived (T) cells. This deficiency appears to be primary, rather than secondary to treatment or manifestations of the disease. Results of in vitro studies indicate that the deficiency is most pronounced in certain subpopulations of T cells, and therefore a disturbance of the balance between subsets of T cells, notably suppressor and helper T cells, may develop in vivo in atopics. Some results suggest that there is indeed a relative deficiency of suppressor T cells in atopic diseases, and thus the hyperproduction of IgE which is associated with these diseases may be explained. The cause of the T cell deficiency in atopy may be a basal cellular abnormality, manifested e.g. as increased sensitivity to inactivation by physiological substances, notably to agents which increase intracellular levels of cyclic AMP. It is equally possible that the T cell deficiency is a direct consequence of subnormal production of certain thymic hormones.