In the present study, the morphological aspect of the skin and the hairs of athymic, macroscopically nude mice (NMRI, nu/nu) was investigated by descriptive light- and electron-microscopical methods and compared with the appearance of the skin and the hairs in normally haired mice (NMRI). These morphological studies revealed that athymic, macroscopically nude mice are not at all hairless, but have about the same number of hair bulbs, embedded in the hypodermis, as normally haired animals. However, within the hair follicles of athymic mice, the keratinization processes are obviously deeply impaired, resulting in the formation of short, crippled and bent hair shafts which only seldom emerge from the hair follicles. The cuticles of the inner root sheath and the hair are not built up, the cortex of the hair being composed by abnormal globular aggregates. The epidermis shows similar disturbances of keratinization, which are reflected by the presence of only few and thin bundles of tonofilaments in the basal, spinous and granular layers of the epidermis and, in the stratum corneum, by bizarrely formed and irregularly arranged lamellae of corneocytes, separated from one another. These results demonstrate that athymic, nude mice are not hairless but that the development and differentiation of hairs are severely injured in this mouse mutant. Analogously, the keratinization of the epidermis is also impaired. In view of the previously shown ectodermal defect as primary cause for the dysgenesis of the thymus, it seems to be probable that defects of the ectoderm are actually the common reason for both the thymus dysgenesis and the severe disturbances of hair development in 'athymic, nude' mice.