Fanconi's anaemia (FA) is an autosomal recessive disorder characterized by a high frequency of ‘spontaneous’ chromosomal aberrations¹ and an increased risk of cancer². If, as seems plausible, the microscopically visible chromosomal aberrations in this disorder result from DNA or chromatin damage that would normally be repairable, the questions arise as to which step (or steps) in the repair process is deficient and whether the deficiency is intrinsic or the result of secondary factors. We report here that the frequency of chromosomal aberrations in FA lymphocyte cultures is positively related to oxygen tension and suggest that the site primarily affected by the FA mutation is in the complex system of defence (that is, protection and repair) against the genetic toxicity of oxygen.