To reassess the use of the linear-nonthreshold dose-response model in the light of advancing knowledge, the National Council on Radiation Protection and Measurements formed Scientific Committee 1-6 and charged it to evaluate the evidence for and against the linear-nonthreshold dose-response hypothesis without reference to any associated policy ramifications. To accomplish this task, the Committee reviewed the relevant theoretical, experimental, and epidemiological data on those effects of ionizing radiation that are generally postulated to be stochastic in nature (i.e., genetic and carcinogenic effects). From its review of the data, the Committee concluded that the weight of evidence suggests that lesions that are precursors to cancer (i.e., mutations and chromosome aberrations), and certain types of cancer as well, may increase in frequency linearly with the dose in the low-dose domain. On this basis, the Committee concluded that no alternative dose-response model is more plausible than the linear-nonthreshold model although other dose-response relationships cannot be excluded, especially in view of growing evidence that the dose-response relationship may be modified by adaptive responses, bystander effects, and other variables.