According to the pathological observations on BALB/c mice after being given steroid hormone, the following results were obtained: secondary thymic atrophy could be observed 24 hours after steroid therapy. The degree of atrophy correlated with the amount of medication and duration of therapy; the weight and pathological changes under light and electron microscope examination almost returned to normal 7 days after the drug was stopped except in individual animals. Our study indicated that secondary atrophy of the thymus can be reversed after elimination of the pathogenic factor.