An individual suffering from ataxia-telangiectasia (AT) presented an acute lymphoblastic leukaemia. He comes from a family in which, in addition to the AT which affects half of his siblings, there are a consanguineous marriage and numerous cancers. In connection with the present case, the authors discuss the main theories as to mechanism of the undoubted facilitating effect of AT on cancer. This syndrome is regarded as degenerative malformation of the lymphoid and reticular cells or as an immunological defect.