Fibroblast cells derived from a café-au-lait spot and normal-appearing skin of a neurofibromatosis (NF-6) patient were studied for radiosensitivity in comparison with two normal cell lines used as "controls." No difference in radiosensitivity was observed between the patient's cell lines and the controls using acute gamma-irradiation. However, a markedly increased radiosensitivity of the fibroblasts obtained from the patient's skin of normal appearance was demonstrated after chronic gamma-irradiation. The cells from the café-au-lait spot showed intermediate sensitivity to chronic irradiation as compared with the control cell lines and the fibroblasts derived from the normal skin of the patient. These results showed the usefulness of chronic irradiation in detecting increased cellular radiosensitivity which may result from a unique DNA repair defect in an NF patient. We suggest that enhanced genetic changes in radiosensitive NF patients may lead to formation of café-au-lait lesions and certain tumors. Such a transformation may be associated with production of radiotolerant cells.