Dorsal skin reactions to continuous topical treatment with different types of corticosteroids were histologically investigated in hairless descendants of Mexican hairless dogs. The preparations tested were prednisolone (ST-1; weak), fluocinolone acetonide (ST-2; moderate), diflucortolone valrerate (ST-3; strong), and mometasone furoate (ST-4; very strong). Grossly, the sites treated with ST-3 and ST-4 showed moderate inflammatory reactions. After completion of the corticosteroid treatment, both sites were less pigmented and had a thin texture. The severity of histologic changes in the skin was dependent on the efficacy of the corticosteroids. The epidermis was prominently thinned from 1 wk after treatment with the corticosteroids, resulting in a flat dermis-epidermis junction. By the end of the corticosteroid treatment, these lesions became progressively more severe. At 2 wk after completion of topical treatment, the epidermal thickness in the sites treated with ST-1 and ST-2 began to return to normal values, whereas the epidermis of the skin treated with ST-3 and ST-4 became thinner. At 3-4 wk after topical treatment with ST-3 and ST-4, the dermis showed hyalinization of collagen bundles. These dermatologic findings in hairless dogs are in accordance with steroid-induced skin atrophy of human beings. These results suggest that the skin of hairless dogs responds sensitively to topical corticosteroids and that these animals are a useful model for investigating the efficacy and adverse effects of cutaneous topical corticosteroids.