Grenz rays (ultrasoft X-rays, Bucky rays) have been used in the treatment of benign skin disorders for more than 60 years. The mechanism of action, the clinical effect, and the potential carcinogenic effect have been mainly unknown, and many of the reported studies are obsolete today. In the present work these concepts have been studied by means of immuno-histological methods, transmission electron microscopy, patch testing, clinical evaluation and epidemiological methods. The results can be summarized as follows. The number of Langerhans' cells (OKT-6 positive cells) decreases in human epidermis after grenz ray radiation and this reflects a true disappearance of the Langerhans' cells as shown by electron microscopy. Grenz ray treatment was found to--suppress the expression of nickel allergy in sensitive individuals--have a tendency to suppress irritant skin reactions--have a very good effect on psoriasis of the scalp. Grenz ray therapy cannot be excluded as a risk factor in the development of non-melanoma skin tumors, but this risk factor is small, if any, when certain therapy recommendations are followed.