A very high incidence of cancers (10%) is recorded in patients homozygous for ataxia telangiectasia (AT). From 1970 to 1987, 35 children were investigated in the department of pediatric neurology in Lille. Three developed a malignancy (one hepatic tumor, one Hodgkin's disease and one non Hodgkin's lymphoma). Constitutional chromosome fragility and immune deficiency are the main features of AT. The first one is probably linked to the pathogenesis of malignancies. Moreover, cancer therapy has to take these features into account.