Summary Features of “runt” disease have now been described in a variety of circumstances. The pathologic events characterized by lymphoid hypoplasia, immunologic deficiency and excessive mortality which result when newborn mice are given estradiol are similar, but not identical, to those of wasting encountered under these other circumstances, and, as a consequence, place estrogen-induced “runting” in the family of wasting diseases. Estradiol-induced thymic involution, an early finding, may be crucial to the impaired development of the peripheral lymphatic structures.