Ataxia telangiectasia is a human autosomal recessive disorder characterized by progressive degenerative changes in skin and the central nervous system. Affected persons have an increased risk of malignanc- and demonstrate marked sensitivity to X-rays when used in cancer treatments. Cultures of skin fibroblasts also show an increased sensitivity to X-rays compared to normal controls. This sensitivity is thought to represent a DNA-repair defect which may be related to their increased risk of cancer. Recently, presumed heterozygotes were reported to show increased sensitivity to X-rays under hypoxia. We propose to investigate the findings of homozygotes and heterozygotes of increased X-ray sensitivity.